Pool heaters can benefit from periodic cleaning.
Gas Heater Cleaning
Most burners lift up from the end that is away from the gas manifold ( usually fitted into a slot at the far end) and push away from the gas manifold to remove off of the orifice on the manifold. They can then be cleaned with a wire brush and either blown out from the top slots or tipped towards the end that sits on the orifice to remove internal dirt.
Also check for spider webs at the end that sits on the orifice. Depending on the ignition source( spark/hot surface ignitor), you must be careful removing the burners.
If you have a silicon carbide hot surface ignitor, they are very fragile and susceptible to failure from bumps against a hard surface and even oil from your hands causing it to fail. Only handle from the white ceramic insulator at the end that the wires are attached to.
Orifices can be cleaned out with a pipe cleaner to remove spider webs or other blockages.
After the ignition source and burners are removed you will need to look up through the heat exchanger with a mirror and flashlight to see if there is any black carbon/soot restricting the flow through the heat exchanger. If there is, the heater would need to be disassembled and cleaned to remove the blockage. This should probably only be attempted by a professional if needed.
Most HVAC companies will work on a pool heater, but not all HVAC technicians are created equal. If needed, look for a company that has NATE certified technicians ( this applies to HVAC service also)!