Quarter bumpers rubber

I purchased replacement rubber channel section for the bumpers from Superformance and tonight thought i would have a break from the dirty work and treat myselfe to something nice to do! however i found that the new rubber was stiffer than the original (more pastic content?) and that it did not fit so snugly to the bumper. Any body else had this problem/got any ideas? Should i have soaked it in hot water first?
Thanks in advance G.P.