Keep looking
First of all, you're genuinely concerned about it, so we already know you're not just giving up. Good for you. Now some tips to maybe help. If there's snow on the ground, a flourescent light makes blood show up very brightly. Next suggestion is to look at knee high brush, too, not just the ground. Wounded deer often brush up against stuff and wipe blood on it you'd miss if looking at the ground only. Third tip is to let your feet take over. Wounded game will often run the path of least resistance and thinking about it too much will take you to "logical" places rather than "easy" places.
Now for my disclaimer. It's a fact of life that once in a while something will get lost for good. Keep up with the old college try and give it your best shot. If, however, it's gone despite your best efforts, keep in mind that you tried and let that guide next years shooting. We all hope you find it and wish you luck in tracking.