There are several issues that could be affecting the proper function of your heater such as a clogged filter, a leak or clog in the fuel line, corrosion, etc. The heater is the most complex piece of pool equipment so a professional evaluation is the ticket. We have over 30 years of experience repairing and replacing pool heaters.
Signs that your heater may need repair or replacement include:
Water leaking from the heater
Dark exhaust emitting
An odd whistling noise
The pilot frequently needs to be re-lite
Never achieving desired temperature
A full replacement might be needed or it could be just a simple repair. We repair or install all major brands: Pentair, Jandy, Hayward, and Raypak.