Hello Dawson, welcome to the Cameralabs forums!
All rechargeable batteries lose their charge over time when not used, so it could be that your AAs have just drained themselves. For the maximum battery life, you should always recharge just before you want to use a battery, such as the night before going on holiday or to a wedding.
Some rechargeable batteries also suffer from 'memory effect' which means if they're not fully discharged first, they won't be able to recharge-up as much. This hardly applies to Lithium Ion batteries, but it does to a certain extent with NiMHs. So before recharging your NiMH AAs, try and run them right down first, perhaps by taking a load of photos in a row with the flash.
It may also be that your particular NiMHs are getting on as well. Even if you take care to fully discharge them before recharging, NiMHs will still become less effective over time and need replacing eventually.
If yours were new though, I'd guess they've just lost their charge through not being used. Run them down, then fully recharge them and try again. If that doesn't work, try a different brand of batteries.
Let us know how you get on.