Often errors will be logged so I suggest you train yourself to always check your logs for clues first. If no errors are logged check your squid.conf for the debug_options directive. Setting it to "debug_options ALL,1" could make Squid emit more nfo (temporarily because you probably don't want more disk I/O than strictly necessary). If that isn't enough you could tune individual debug sections by section code
like debug routines, disk I/O, like that.