Back in '08 Glock released a Wearable Parts Replacement Schedule for LE armorers for the Gen 3 & older .40's in LE service.
The recommendation for the mag spring replacement in the G27 was 5,000 rounds, or, for best results, replacing the mag springs at least every other time that the recoil spring assembly is replaced. (Replacement of the RSA was listed for 3,000 rounds in the G27, or whenever testing at each range or qual session, using the Glock recoil spring field test, might indicate it was needed more often.)
The latest Glock armorer manual includes language which describes how any spring can be damaged, weakened, worn or broken and should be evaluated often. The mag spring is one of the 9 springs listed that may require attention at some point, as they age and "tire" (their wording).
I've listened to different instructors over the years describe different ways to check and decide if mag springs are starting to weaken and may require replacement. Manually running the slide in a brisk manner (obviously using an EMPTY gun & mag) to make sure the follower consistently locks the slide back on an empty mag is a commonly discussed method.
Seeing feeding failures at either the end of the mag load, or approaching the end of the mag load, is also a common sign (presuming the mag is clean and not just contaminated with fouling, debris, etc).
Using a new exemplar spring to which used springs are compared; how many spring windings may stick out of the mag body with the floorplate removed; and how far the empty mag "pops up" out of the grip when the EMPTY pistol is being bench checked (turned upside down & mag release pushed to release mag) have also been discussed in class (as different armorers have come up with their own ideas).
Before I'd attribute failures-to-feed to a mag-related condition, I'd consider the more commonly encountered influences of shooter & ammunition-related factors.
I don't fire my G27 as much as you do (spreading out my shooting among a number of different handguns), but I'm coming up on close to 11K rounds fired through it. I've experienced perhaps a dozen or so feeding issues and stoppages over the years I've been using my G27. The apparent causes seemed to have included shooter, ammunition and apparent follower issues. The shooter related issues occurred when I momentarily (albeit unintentionally) relaxed my grip/locked wrist.
I consider my G27 to be perfectly reliable for use as a retirement weapon, BTW. (And as an off-duty weapon prior to that.) I work hard to prevent any shooter-related issues from occurring during qual, training & practice sessions which might involve movement, the use of 1-handed and non-dominant hand shooting, as well as using various cover/barricades while shooting at threat targets. I know I start to get tired after shooting upwards of 200-400 rounds during any particular range session, especially if done through a couple or even several guns.
If you're shooting 2-3 hundred rounds per range session it's not surprising that a shooter-grip issue might momentarily surface due to hand/wrist/forearm fatigue.
Are you using the stock 9-rd mags, just out of curiosity?
Just my thoughts ...
What ammunition & bullet weight are you using when you encounter your single failure-to-feed (or were using just prior to experiencing it)?
Do you clean & inspect your mags (especially considering how much you shoot each month)?
