The 28BYJ-48 is a unipolar motor which has 5 wires. The L293D is a bipolar motor driver. A bipolar stepper motor has only 4 wires. I think you have an error in your connections. The 5th wire that is the center tap between the two coils should not be connected to ground. It should be left unconnected, or better yet, the trace on the 28BJ-48 that connects the two coils should be cut. Doing so, turns the 28BJ-48 unipolar motor into a bipolar motor. If you google this, you will find instructions on how to do that.
