Can you please send me some model of gate-drive transformer, or a high-side mosfet driver circuits please?You're only driving the high side mosfets with 15 volts, so the highest voltage that the source terminals can get is 15 Volts minus the G-S turn on voltage of the mosfet. For the IRF510 that is 3.8 volts. So your source pins are only getting to about 11 volts.
If you want the source pin to reach the positive rail, you need to drive the gates with a voltage higher than the positive rail. This is typically done with either a gate-drive transformer, or a high-side mosfet driver.
What is the purpose of your K signals?