Is your motherboard mis-aligned? On the standoffs (if applicable) correctly?
If you remove the motherboard from the chassis, does the card go in?
There are some tolerances involved, it could be you got a slightly larger card and the slot is slightly under spec. Only seen that once with some mismanufactured memory slots, and the board manufacturer was well aware.
The graphics card will have a tab that extends down into a gap between the motherboard and the back of the case. Try loosening the motherboard hold down screws a bit.
that should allow the motherboard to slide back a touch and allow the graphics card to fit.
later, you can tighten the motherboard screws.