Raypak Heater p-r156a ignites but doesn’t get hot

Hi, I have an 18 foot diameter above ground pool with a Raypak 156a heater which worked fine most of the summer but now when it ignites I can’t hear the fire like I used to. I can see a blue flame through the little window inside the heater but the flame looks weak and no heat can be felt on the outside of the heater. The heater thinks it is working fine, display says spark and then heating like normal. I’ve cleaned all filters, chemistry is good. Any thoughts on why it would do this? Thanks in advance for any help.