Not 100% sure what you mean, but if you're meaning when the roof is closed, there is a gap between the roof and the seals above the windows (especially at speed) then this is usually down to a failure of the cables with-in the edges of the roof. The cables hold the roof down at the edges to keep it taught.
Over time the cables can become stretched or even break inside the fabric and require replacing. You can find them for about £20 on Ebay.
I replaced mine 7 years ago when I had a Mohair roof fitted, and on the passenger side, it is still nice and tight, but on the drivers side evey so often, it comes loose and requires a tighten (this side has just stretched, not broken).
If your cables are only stretched, then you can open the roof 90* until it is vertical and stop it there. In the top corner that would normally be by the rear window when closed, there should be a silver bolt type thingy hiding up inside the fabric/metal of the mechanism. Firstly, you can try and tighten the nuts a bit more and see if that works as they can become loose over time.
If that doesn't work, if you take the retaining nuts off (try not to loose them down the gap between the body and interior - a towel/rag helps to plug the hole), and pull the bolt out of the housing, you can then tie a knot in the cable, put it back in, then tighten the nuts back up - don't fully tighten them though until you have closed the roof and checked them to gauge the correct amount of tension to put on. I have found that they should be about 1/2way to start with, then open and tighten accordingly until the roof closes and locks whilst pulling the edges taught but without too much force required.
If the cables are broken, when you take the bolts out, give them a pretty firm tug, and if it comes out then you know they are broken.
To replace requires a similar procedure, but you have to peel the fabric of the roof away at the front. Start at the bit where it would be inside, above the A pillar when closed, and peel it around and you should see the little clips that the cables clip onto. Reomove the old cables and thread the new ones through the side of the roof and bolt them back in, and re-glue the roof back in place at the front with some strong glue - I used some stuff from Wickes and it worked fine - can't remember what type it was though now !!.
My drivers side started with new cables at about 3/4 tight, went to full tight, came loose again, took the bolts out, tied a knot in it, started at 1/2 again, then to full, and have just gone to the 2nd knot and 1/2 tension again !!. That gives an idea of how much the cable can stetch without being broken !!.