Occasional cranks ok but won’t start is almost always some type of intermittent fuel or spark problem. First step is to narrow which of those it is, spark or fuel. Ask your shop if they can somehow outfit your car so when it happens you have a clue to whether it is spark or fuel. Beyond that you option is either to just make a guess and start replacing parts, or park the car at the shop and they can drive it as their daily driver or to run errands and eventually it will fail while one of the shop techs is driving it.
If I had that problem on my Corolla I’d bring along a spark testing gadget. If it wouldn’t start and the spark was ok, then I’d know it was probably a fuel problem.