Hi Derek,
I had a similar thing this trip with the same top screws of the blinds the rear kitchen blind, I used a slightly larger diameter screw of the same length and was able to tighten it up, I think what you may be experiencing is the point where the new self tapper has reached the maximum diameter of the existing hole and will need a bit more torque to cut its way into the steel frame enlarging the hole.
Btw, don't you just hate Phillips/Posidrive screws, so much better if we could get them in either hex or torx, no more slipping then! Perhaps someone knows of a supplier?
Chris

Cheers Chris. Not convinced there is metal back there now. Soon off to Screwfix to check if they have very small cavity thingies or the Evotik product

These brackets. Are Seitz as used by Eriba? I have quotes from Jandi and AL at a stupid figure + PP for each.

They're generic Derek. Go to Leisureshopdirect. Ian

Have ordered thanks

Awaiting delivery and itching to get on with replacing brackets and get another thing off the list.

Arrived today, will aim to fix maybe Thursday as still on GD duties

All done. GD had a party to go to this afternoon, so got on with it. The odd one was the bottom right of the LH blind where there seemed to be a chunk of metal and screws would not go into it (tried a self tapper and no good) and the space behind the inner face wasn't enough to get a full wall plug in.
In the end I cut down a cavity wall plug short, jammed that in and then screwed into that. Seems OK.
Just have to pay Jandi now.