i have a 1988 Base Camaro with the 305 TBI motor

i bought the car about a month in a half ago, i brought it home after getting it jumped, and charged the battery to full, and all was good, i could start it up after 3 or more tries

about a week later i took it to the shop and got a tune up/sensor replacement and airfilters and stuff replaced ect, and the car would start up on a dime, no problem.

i then put it in my grandmothers garage and didnt touch it until i got my lisence this week. so it was sitting there for about 3 weeks, with no one touching it. at my grandmothers house 2 days ago, it wouldnt start, so i charged it for a good 20 minutes, then came out, and it started up, i went and filled the tank up and parked it outside my house after driving it for a bit.

the next day(this morning) i went to start it up to get to school, and no luck. after school i let it charge to about 85% battery power and it didnt start.... i tried to jump start it with a portable jumper and that didnt work. The ENGINE IS TURNING OVER but not fully starting. i dont know if its a battery problem anymore, any suggestions.