It is possible that some plug fouling may occur with certain PA28 high time engines following protracted taxying. There is a well know cure for 'burning off' this fouling, but it must be very carefully followed to avoid engine damage.
Basically, it consists of setting full power, then leaning the mixture for a short period, before setting it fully rich again and then throttling back to 2000 rpm to repeat the magneto check.
DO NOT DO THIS UNLESS YOU HAVE BEEN TAUGHT THE CORRECT METHOD!!