Here's some grown-up-newbie pointers of what may be going on...(oh god...the brain cells I've wasted with this stuff
)
1. If you've left your battery outside in the garage...it'll be cold, and cold batteries don't work well. Try to keep the batteries at room temp.
2. As said, a smart charger is a charger that will detect what the cells of the battery pack are doing, how much charge they need/when they're done. "dumb" chargers just dump power into your pack at a pre-set rate...and maybe they'll shut off
3. Just like any rechargeable battery in your camera/radio/flash light/etc...they don't last forever or "charge up" like when they're new. An "old"/"tired" battery may seem to be charged, and may seem to work at first, but just doesn't last long. Nothing to fix...chuck the pack out and buy a new one. A shitty charger can dramatically shorten the lifespan of a battery pack.
4. Your gun doesn't have any electronic circuits in it...only a few do. The on/off switch in your gun is about as primitive as it gets. Basically there's a break in the wiring (two copper fingers) and a metal wedge that gets shoved between them to complete the circuit when you pull the trigger. So...while you should have a battery drain if you leave the batt plugged in...it's ALWAYS a good idea to remove the battery when you're done. A dummy might shoot themselves...the batt may leak and get acid in the gun...etc..
And...unlike a firearm...you can't actually prove/clear an airsoft gun like you have. You can take the mag out and fire a bunch of blanks...but it's not unheard of for a BB to be stuck up in the feeding path and get fired out even though the mag is out. Removing the mag...inverting the gun (so gravity helps a stuck BB go into the feedpath)...and firing a bunch of semi-auto blanks is about best you can do to clear the gun. Removing the battery is 100% safe...since it won't do anything without a power source.
Have fun...wear shatter resistant goggles