HQ, this is a wild guess but your scenario sounds a lot like ear fatigue. I don't know how you make beats but the more you listen to your beat (as you're making it), the more your ears will tend to hear what your mind wants them to hear. If your beat sounds great while you are making it then you stop and come back even a few hours later and it doesn't sound the same it's because your ears are tired and your brain has taken over and started making corrections. If you've never heard of this, I'm sure it sounds kinda crazy but this is why Professional Mixers and Masters take big breaks while working on a song.