If you can't see the water stream from the returns . . . it doesn't tell you anything. The DE is either on top of the sand in the filter, and will be backwashed out OR it's sitting on the bottom of the pool along with the dead algae.
*If* the DE is on the filter, you should see filter pressures rise much more quickly than normal. If this is not happening, I'd recommend opening one of the filters and checking sand levels.
Something just occurred to me -- if someone has backwashed the filters one at a time, instead of doing both simultaneously, odds are you've blown much of the sand out of the filter. I'm guessing the flow curve below shows your pump on the "F" curve. If so, that pump will deliver WAY too much water, during backwash, for a single filter. Maximum backwash flow on a TR-140 is about 110 GPM. That pump can deliver 2x that rate of flow.
If the standard practice at your pool has not been to backwash both pumps simultaneously, you'll need to (a) replace the missing sand and (b) develop a process that will ALWAYS be followed during backwash, in order to prevent excessive backwash rates.
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