I think our strings are too short, they have knots in them with constant use over the years, I have bought new string to put on when we go out next time.
I think because ours is a really old one its slightly different, we dont have any buttons to put the strings on, there are hook things underneath which we think we put the string on?!7
Our green one has little sharp hooks also, mind your fingers - they can drawn blood.
Be very careful when opening the low bed side, the hooks where the roof attaches to when it's up can catch on the inside of the roof when it's being opened, it took us a few goes to master the art.
Our strings are just long enough so the bed boards pull the U bars into a vertical position, does that make sense? Have you polished the inside of the roof with a silicone based product? It makes it slide over the metal U bars really easily.
When we first had ours the roof gripped the U bars, and it was a nightmare to do the roof, the previous owners were 6' plus and put the roof on after with out any problem, me and OH are at least 10" shorter and we struggled, sweated and swore - until we had the silicone tip. Thanks Helen.