Clean the probe with deionized water that has the pH lowered to 3 using HCl. This would clean any coagulant nanoparticles from the probe in case those nanoparticles are interfering with the measurements
call the fluoride probe vender to see what cleaning steps they suggest
investigate options at VWR. search for Fluoride electrode
It looks like we will have to pay something between $600 and $1000. I am concerned that given how we use the probe that these expensive probes won't last any longer... So Ideally we figure out what is required to care for a probe so that it lasts longer!
Proposed steps