They can get pretty tight. I take mine all the way out and clean the threads when I take the cover off in the spring because I've had them stuck before.
I'd put penetrating oil on them and tap them with a hammer. When you start to take them out, tap the wrench with a hammer gently to try to start the threads moving. Torch would be the next step.
I had one where the insert was apparently bent during installation and I never could get it to work until I put a new one in that hole.
They have ridges, and they are driven in with a special tool. If the holes are oversized from the drill moving the ridges may not engage. I put epoxy designed for masonry and metal in the hole before tapping my replacement in. It's holding fine so far.
Wd40 and a new hex
Tool helped. I got most of them lose except for five which spun out of hole. I manually loosened a few with hex wrench and vise grips but might need to buy a couple new ones.
I had three seize up on me a year or two ago. The whole thing came out. I used quickrete anchoring epoxy to put the new ones in since they wouldn't grab when I put them in. I rinse the holes, and use wd-40 type lube spring and fall, and they are always pretty easy to get out. Didn't know about the drill bit. That'll speed things up when I close my pool.
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