Your problem can vary depending on the brand of refrigerator.  Is it a Dometic ??  Also, (this may sound stupid) are you trying to ignite with L.P. while still plugged in to AC??  If so it will not happen--there is an over-ride that will automatically revert to AC rather than L.P.  If this is not the case, check your L.P. tanks and regulator.  Different brands have different remedies--so it would help to have your brand and model number.     Zane.

First...make sure tank(s) are full and turned on. Next...make sure the selector switch on fridge is turned to LP or gas. If the selector switch is on DC or electric, it won't light.

If after checking / trying the above it still won't light,open the fridge panel on the outside of trailer and check to see if spiders have clogged up the tube that supplies the gas to the unit with a spider web, this can be the problem sometimes.