I got some proper strawberry feed, it's like a powder (dunno what it's made of exactly). It said put it on when they are flowering. Seemed to work for me! Maybe you put it on during the leafy period instead?
I must give my strawbs a trim this weekend, now that the big cold spell is subsiding. They are all shrivelled up poor dears but in a week or two, they are bound to be starting to regrow again. I usually attack them with scissors, cut off all dead leaf growth and then brush down the bed (it's on plastic) with some twiggy branches to clear them off.