Soft-top Tight to Erect
Soft tops stretch in use, so hood manufacturers and fitters set them up to be tight initially - if they didn't the hood would turn into an ugly floppy bag of a thing once it had been used a bit. However, the converse is that they then shrink back while not used, and as a result it is very common for tops which haven't been up for a while to be difficult to stretch into place. For that reason, I usually erect the top when I put the car in the garage and may not be using it for a few days.
Fortunately the shrinkage is reversible, and reversibility is shown well with the packaway hood on the 1963 MGB: After a summer when the hood stays off all the time (because I don't remember my own above advice) it's tight to get the hood to fit even when the rear tensioning bar is collapsed into the forward position. So the first couple of times I use it (looking ugly) with the bar in that position. With use, the hood starts to stretch again and the bar can be pushed backwards into the correct place. With further uses, the hood can be removed for the odd sunny winter day and then replaced with the tensioning bar remaining fully extended.
Folding frame hoods present more of a challenge, as they don't have a tensioning bar and can be a real struggle to get the header rail up to the windscreen if the hood has shrunk back a lot. If the top has been left down while the car was parked for a while and is too tight to erect, a way of overcoming the shrink-back is to warm the vinyl - either by leaving it out in the sun or (if the weather is unhelpful) draping the soft top over the frame and placing a blow-heater inside the car to warm it up. Once it fits take it for a motorway run and the aerodynamic lift on the top of the hood will finish the job.
Don't be tempted adjust the hood to get it to fit easily when the weather is cold and it's been off for a while, especially by releasing it from the header rail, as once it's been used and stretched again it could become floppy and horrible if not done properly.
(Written by Paul Walbran - www.mgparts.co.nz)