die hard

2015 gmc terrian the window goes down using the switch but wont go up using the switch . i put 12 volts right to the motor regulator goes up and down fine .but wont go up with the glass hooked to regulator ? if i pull up on glass it goes up slow you can here the motor struggle . with out the glass hooked to regulator glass slides up and down smooth . both regulator and glass work smooth but just not together ? ive tried keeping bolts that attach glass to regulator loose thinking its fighting its self and still wont go up? i grabbed a hold of regulator with my hand as it going up try to stop it has more power than i can hold back .either any way i try it wont work off switch ? im lost looking for some advice hope some one has dealt with this problem before and can help .thanks