Pump Not Priming Or Sucking Water

[arbutler11]

We have had our inground pool for 16 years. We have always done the work on it ourselves. It is a 41x21 inground pool with two skimmers and main drain. We have a Hayward DE75 filter which is original and a Hayward 1HP Super Pump which we have had to replace every 4 or 5 years. We replaced it two years ago.
The other day the pool was running fine until we hooked up the suction hose and started cleaning the bottom of the pool. We had to change the DE but the pump would not run if we had the flow valve set so for both the skimmer and the main drain flow.
We thought the skimmer was plugged so was unhooked the flow valve and put a blower to it. Water came shooting up the skimmer so that wasn't it!
It runs if we have the flow valve set to the main drain or the skimmers but not very well and not at all if we set the flow valve for both.
We thought it might be the impeller so we had an old Super Pump and we took the impeller off of it and put it on the pump we are using. We took the fingers out of the DE filter and hosed them off really good and replaced everything. Still the same thing.
When it was running good, there was no leaks in any of the piping.
What did we do wrong and what can we do? We really need help.

[tenax]

2 seasons ago, my pump, which works fine, wouldn't suck water for anything..it ran ok..but would not build pressure. i was pulling my hair out to the point given i had checked all the lines and fittings. i ended up getting the pool guy to come in and check it out..and here all it was was i needed a new lid for the basket on the pump..the seal had gone and even though i had looked at it , i couldn't see it had a break through part of it..that let enough air in at the pump itself it could not even build pressure to suck in the water. he replaced the seal on the lid for 10 dollars and 50 dollars for one hour labour and fine since.