Concrete screws not going in all the way
That's far too long a screw for that application. 2-3/4" screws are designed for mounting 2x4 lumber, for example, which would leave just 1-1/4" of penetration or so.

Concrete screws, especially thinner ones like that, should only engage the concrete maybe an inch or a bit more. If you try running that in with a stronger drill it'll almost certainly snap. 

Try the 1-1/4" screws, and be aware that running them in more than once usually results in the threaded concrete failing due to the grinding effect of insertion. They should be a once-and-done install. Move to a new hole if that one doesn't seem to hold with a shorter screw, or use some epoxy to rebuild the bond.