Spiders and other pests like to hide in heaters and can easily clog up the system. I would shut off the gas, pull it apart as best you can and get a shop vac or air compressor and clean out everything you can. The other issue could be a thermocoupler going bad and since you don't say what model it is there could be other issues associated with electronics or circuit boards.
If the igniter is sparking, then the board is happy that the safety switches are satisfied. In this case you need to see what voltage you have at the terminal from the board to the gas valve.
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