Mine does the same, closes ok but then lets go on one corner, it needs a push up inside to lock it down, I now have a little tool to do the job easier, just a short length of metal tubing with rubber stops on each end, I find this much easier than having to get in the back and do the Charles Atlas thing.
I think I need to fashion a similar tool.
I do weights 3 times a week at the gym, but had to give it everything I had yesterday to get the strut to lock-out.
It's never really been an issue before, so hope yesterday was a one-off.
I'm still experimenting with different ways of finishing the manual procedure - locking down the rear deck via the boot before bringing the rear screen down; and bringing the rear screen down and then locking the deck via the boot.
Maybe the stage at which I try to lock-out the strut has an effect on how difficult it is. I'll find this out after a few more goes.
It also seems strange that only one of the struts needs to be forced back (over-centre), in my case the passenger side.
There's a little bit of movement in the driver's side one, but its position can't be shifted and it does seem to be sealing.
So now I've more or less mastered the full manual opening and closing of the roof, I'm finding life with the Spider much more enjoyable, and it only takes slightly longer to open and close the roof than my previous system of part electric, part manual procedure.
Physically lifting the hood out of its storage space when closing is the trickiest bit - it is quite heavy, but doable.
I now intend to have a lot more open-top motoring over the coming months, and with no hint of the dreaded roof anxiety!