A damaged rubber tier, worn wheel bearings, loose blade tension or a dull saw blade can cause the blade to slip off the wheel. Check blade tension by pressing in on the side of the blade using a gloved finger. Tighten blade tension if you can easily push the blade in more than 1/4-inch.
If blade tension is okay, a dull saw blade could be causing the problem. If the saw blade is dull, you have to push the work piece with more force to move it through the cutting area. The extra force could push the saw blade off the wheel. Replace the saw blade if it's dull.
If the saw blade is sharp and the blade slips off the wheel even when you only use light pressure to push the work piece through the cutting area, check the rubber tires for damage or wear that could lead to the blade slipping off the wheel. Replace worn or damaged rubber tires.
If the rubber tires are okay, manually spin the wheels and check for wobble. If a wheel wobbles while you spin it, replace its bearings because they're no longer supporting the wheel enough to keep it from wobbling.