Be sure the rifle is clean. Then check to see that the gas rings on the actual bolt are properly spaced at 120 degree angles.
Then start looking for other reasons if it continues.
What most folks call short stroking is when the bolt fails to go far enough to the rear to pick up a new cartridge from the magazine.
Some times the bolt goes back far enough to allow clean extraction and ejection but not far enough to pick up a new round from the magazine. Your standard GI immediate action drill can allow you to fire again.....and may then be required between every round or not until you have time to break down the rifle and look for causes.
Some times the bolt comes back far enough to start extraction, and almost enough to allow ejection but the partially ejected round does not clear the ejection port. These can be the worst as the returning bolt can really bend up and crap the spent case into spots cases were never meant to go and once the cartridge rim clears the extractor, clearing the stoppage can be a bear. Immediate action drills frequently only make the issue worse......thus the importance of looking in the chamber in the midst of the Immediate action drill.
The last is where the bolt only comes back far enough to start the case out of the chamber, but not far enough for the mouth of the cartridge to clear the chamber and the bolt then comes back forward to push the fired case back into the chamber. As the extractor still has the rim of the cartridge the Immediate action drill usually clears this, though again the problem may not go away until you have the time to do a thorough inspection cleaning and lubing.
In the service I found three issues that were the main causes of short stroking. The easiest to notice was sticky black carbon crude so thick around the bolt carrier that it slowed th ebolt down so much in both directions that the rifle was about useless. This was mainly caused by petroleum lubricants, and using too much of them, high humidity and dust/dirt.
Nest was improper or lack of cleaning the outside of the gas tube wher it protrudes into the upper and or failing to clean the inside of the bolt key or a combination of the two. In the service we used worn bore brushed to clean the inside of the bolt key and outside of the gas tube.
Althout some still claim it makes no difference the early manuals cautioned to ALWAYS check that the three gas rings on the actual bolt had their slots aligned 120 degrees out when assembling the rifle. If every thing is clean and lubed, yes sometimes a rifle fires just fine with the three slots all lined up.....but don't count o that. Once the bolt carrier starts to meet resistance those gass rings need to stop more gas and I have seen more than a couple of guns stop having failures after those rings were checked and adjusted, despite the assurances of modern experts ( that may not have even been born in 1967 when those manuals were written) that gas ring alignment makes no difference.
The last case of short stroking I doubt any one here will see is the gas tube itself collecting a build up of lime like crud from having used a lot of blanks and the Blank Firing device. With M16 A1s this was a serious issue near the end of their service life. The standard "book" issue was to replace the gas tube. If your Armorer only had five spare gas tubes and you had twenty rifles out of 200 red tagged for short stroking and had tried every thing else what do you do? We more frequently cleaned the crud from the tube using the wire from a spiral wire bound paper binder streatched and straightened just enough to allow it to be inserted into the gas tube and twisted and removed repeatedly until it was fully up to the gas port and the work table covered with lime like crud that had been drawn out. This was followed by the use of the long pipe cleaner that was gently inserted and twisted and removed. It was important to be gentle as there might be a tight spot missed by the wire binder and the pipe cleaner could become stuck and even break off when brute force was used to try to get it out.
Now as to improvement to the extractor spring......usually if the issue extractor is not reliable it is because folks are not using their chamber brushes to keep the chambers spotless and so extraction is more difficult. Not a bad idea to go with one of the improvements, but seeing to proper cleaning and using good ammo is even more important, whether your extractor is powered by just the issue spring, a washer compressed issue spring or assisted by a magic blue or black, whitish "rubber" dohicky.
Of course many of the experts that respond on THR and every other gun board will tell you, despite your experience, that ARs never malfunction, so, again, what do I know?
Before anyone gets mad and asks, yes I do own a commercial AR15, a Colt HBAR, and no that particular rifle with brass cased ammo has not failed me once since 1992, but every other thing I have described I have seen on M16 A1 service rifles and on some commercial rifles.
I will freely admit that the vast majority of AR stoppages are operator error in some way or another......unless the bolt shatters, or the extractor breaks, or the bolt carrier key shears, or the recoil spring tube cracks, or a pin walks or well mechanical stuff can fail in any machine......more in some and less in others.
-kBob
