I would like to know how to make a colar of a sweater more stretchy?

We need a little more information – what pattern, what type of stitch is the collar knit in, etc?

it is a standar pattern(rond) and the stich is k1p1

Saying it’s a standard pattern doesn’t help much; do you have a link to the patter? When you say collar, are you talking about a neckband? If you had to pick up stitches, you might not have picked up enough, so maybe you need more sts. Or use a larger needle on the ribbing.