ATWOOD FURNACE PROBLEM

I have a 1999 Fleetwood Southwind, the problem I have is the front furnace. I turn it on and the blower comes on but it will not click and lite. I have swapped circuit boards from front to back and the back furnace will light with the board from the front furnace. I put the back board in the front and still nothing. The back furnace will lite with both circuit boards. I just changed the electrode in the front and still will not click and lite. What else can I check? Thanks for the help.