wierd problem with tyres

[moggy12]

i race my xray T3 at my local indoor club but i'm a few problems with tyre grip i put a new set of sorex 28 with jb inserts on just before xmas car was great did 3 5 min races then club was cancelled because of snow then xmas break so it was 3 weeks without racing cleaned tyres when i got home left them on the car but on a car stand and in the house came to race last night (thur) no grip at all had to change for another new set what is the best way to store tyres/wheels between races ? as surely grip can't go away that quick thanks for any replies