HV400 is an analog current buffer IC - it requires a substantial amount of input current (at least 15mA) just to even START to turn on, and can require brief pulse of 0.2A input to turn on fully to switch larger gate currents on MOSFET. IT is possible that your PIC cannot source this level of current and you may need to use intermediate logic-level MOSFET circuit to drive it fully.
Also, I see on the datasheet this text
"The input voltage should be 2V higher than the desired output voltage."
And from quick look athe the part schematic, it would seem true. So, unless your power MOSFET will turn fully on with only 2-3V gate-source voltage which is unlikely, you will probably need to use an intermediate circuit or other driver IC to get the HV400 input voltage level up above its outputs level, and corresponding to an acceptable voltage for the MOSFET. Since we don't know what MOSFET you chose or what your power supply rail is, we cannot tell you more, I think.
If you can post your schematic it would be much more helpful to see if this is indeed the issue or if there are others.