What? I had caught an Espurr on low health with a Heal Ball, and I succeeded, but it didn't heal the Espurr! Did I do something wrong? Did I miss anything? Or was it just a... a... glitch?
Please help me!
There's a few possibilities.
It wasn't a Heal Ball. You said you made sure it was a Heal Ball, but you may be wrong. Dream and Love Balls both look similar, you could check if it was one of them you caught the Espurr in instead.
You had multiple Espurrs in your party, and you checked the wrong one. It could have fully healed the Espurr, but you instead looked at an Espurr at low health that was already in your party.
It did heal the Espurr, you sent it out in battle, then it got damaged again. Heal Balls don't consistently heal your Pokemon, they only do it once, when it's caught.
You came across an undocumented glitch. Bulbapedia's glitch list doesn't mention this anywhere. While it's extremely unlikely this is the case, it's possible.
Previously used hacking devices messed something up. If you've hacked your game, it could have changed what happened upon capture, and made it so the Espurr was not healed when it was caught.
You said this happened a while ago. Human memory is not perfect, and is subject to error or mistakes. You may simply have completely forgotten what really happened, and assumed your Espurr didn't get healed, when it in fact did.
if none of these are the case, I have no further possibilities as to what could have happened. Hope I helped!