Bert Coules wrote

I have vertical slatted blinds across two separate sets of patio doors;
the blinds were made to measure just over a year ago and seem to be of
good quality.

So why aren't you making them fix it ?

One of them has developed a fault in the mechanism which draws the slats
sideways (as opposed to the one which rotates them): though the slats move
very smoothly in one direction, in the other they are extremely difficult
to shift, to the point of sometimes jamming completely.

The online repair instructions I've found all suggest that the fault is
with the leading top block, the first one of the bits that slide along the
rail. But the videos are all so badly made and so poorly narrated that I'm
having trouble following them.

I have discovered that the block seems to have more rotational slackness
than the others: if when it gets stuck I move it gently on its vertical
axis I can often free the movement, even if only for a few inches.

This end block is also the only one which has a screw on one vertical
face: for tensioning, maybe? But adjusting it appears to make no
difference.

I'm going to ring the supplier on Monday but wondered if anyone here had
any preliminary thoughts as to what might be wrong and how to fix it. If
it is that end block, can it simply be replaced?

Many thanks.