Ok, those are logic level MOSFETs so you do need the Gate-Source voltage (Vgs) to be 4.5V to fully turn on the MOSFETs.
How much current does the motors draw when run directly from the battery?
What is the battery voltage? (same question guru posted)
I've also designed and built several H-bridges and did get them to work well but its not an easy task but it is a very good learning experience so keep at it.
Do you have an O'scope?
Are you using a SPICE program to model the circuit? This can be very helpful. Linear Tech has a free one available to download called LTspice.
Will you please tell me how to modify this circuit to get max performance.
You will have to make measurements and then determine what to change. Maybe start with just the high side MOSFET and get it to turn on fully. Then do the low side MOSFET working. After that combine just one each of the high and low side MOSFETs an get them working together. Lastly do the full H-bridge.
This is the DIVIDE and CONQUER method as it simplifies the problem into solvable pieces. Once each piece works then combine them into the finial circuit. I use this method in the professional realm in designing hi-tech instrumentation.
Good luch and be sure to have fun learning.