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?