E&H FMR51 ultrasonic level transmitter

I have a micropilot FMR51 installed over a cylindrical open top vessell, scaled 0-12', distance above the water is around 4' below the transmitter, it has a 2' blocking distance dc is 4 ... 7, everything looks great but the transmitter is reading 93% it should be around 80% anyone have any idea what could be the problem?
I attempted to map the transmitter again and ended up with the correct reading but about a minute after finalizing the map it reverts right back to 93% full, there isnt any obstacles over the tank that could affect the reading that i know of.
FMR51 is a radar, not ultrasonic.
Check if the Empty and Full values you set in the configuration wizard are correct
DC of water is 80, set the parameter to "Water based"
If there's a display on the radar, you can see an echo curve on it for checking if there's a false echo at 93%
If there's an obstacle at 93% value of level, perform a false echo mapping in the setup menu
You're measuring water in an open vessel, it's the easiest application: the radar MUST work fine