Hi Peter,
Thanks for contacting Maxim Integrated Technical Support.
I have reviewed your schematic, and I noticed that you connected the VH pin to ground.  The proper connection of this pin is to connect VH to IN through a 4.7uH capacitor.  This regulates VH to IN-5V.  This regulator limits the external P-channel MOSFET gate swing (EXT), allowing high input voltage operation without exceeding the MOSFET gate-source breakdown.  By connected VH to ground, you are allowing a gate-source voltage of -24V.  The MOSFET you have selected can only handle a gate-source voltage of ±20V.  It is possible MOSFET could have been damaged from exceeding the absolute maximum rating.
I would recommend changing your design, so VH is connected to IN as show in Figure 1 on page 8 of the datasheet.
Best regards,
Samantha Morehead
Applications Engineering