Which version are you using ?

Do you have enable_polycollision set at 2 ?

Use:
var enable_polycollision = 2; //(Put above, around your include statements.)
or
enable_polycollision = 2; //(Put in the Main just before level loading.)

Then your polygon flag assignments should work...
And your collision hull sizes should be adjustable...
Let us know if that works!