Don't worry. I'm sure you've got time to get some more seeds in, they'll soon catch up.
I sowed some pumpkin seeds on the 16th of April and out of a whole packet only two came up. I sowed another lot on the 10th of May and every one is up and going great guns.
I don't know what the problem was with the first lot but suspect it was still to cold when they went in.
Good luck, hope you grow some whoppers.
