Update:
My spa heater that was taking a few minutes to ignite and heating the water perfectly as recently as last week will no longer ignite at all. Even after 30 minutes, nothing happens.

Going back to the earlier replies, now that it suddenly won't ignite at all, what should I look for, and how easy is it to get to?

A few years ago, I had a similar problem, and the repairman said there was a tube going from the manifoild to the pressuyre switch which was clogged. Once he unclogged it, it worked fine. Where is the tube and pressure switch located, and is it something that I can safely test and/or replace?

Thanks again...