It always helps to get it hot/warm first.........If you can't let it sit in the sun, use a hair dryer, heat lamp, etc.....and once it is warm, hot, etc....then it should stretch. Also use a vinyl conditioner cleaner on it...keeps it from drying out.
As for using the cover, well I have owned convertibles since 66 (my first car was a 59 Pontiac Convt.) and I learned early on that the best way to keep the winkles out of the top is to always put the top up when it is parked. So I do not even use the cover.....I put the top down when driving around from place to place then when I arrive at my destination I put the top up. That way it stays stretched (which keeps the wrinkles out so it looks nicer), keeps the sun away from the interior, also keeps other things from getting on the interior, bird crap, etc......
Hope this helps