Master Spa keeps showing problem code and not heating.

I have a Master Spa that has been the best spa we've ever had, but the last few days its show code LF which the manual says its persistent low flow problems and shuts heater down leaving everything else working normal. Manual solution is turn power off for 15 mins and back on. this has worked in the past but now only for 10 to 15 mins and starts all over again. Water level is right, jets flow great when on, anyone has any suggestions? Thanks for any help.
Larry

Try removing the filter to see if it goes in to flow. If not replace the filter. If still does, does your spa have a small circulation pump that runs 24 hours a day, if so verify that is running.