Design of a motor SPDT switch that can be triggered by 5 V or 3.3 V digital output
Look at the curves on page 3 of your MOSFET's data sheet:



You are not fully turning on your MOSFETs.  This means the drain to source resistance is higher than you think.  You need to look at more than just the Vgs(th).

If you don't see the phrase "logic level" on the first page of the data sheet, you should scrutinize the conduction curves before trying to drive them with 5V or less.

Possible solutions are to use a gate driver, or logic level MOSFETs to drive the power FETs.