I got this info from bettycrocker.com
"You may not have used enough water, margarine or egg. Be sure to measure ingredients carefully. To measure water, place a liquid measuring cup on your counter, pour in the water, bend down and check the amount at eye level. To measure margarine, use a dry-ingredient measuring cup. Press the margarine into the cup, then level it off with the edge of a knife or spatula. For best results, use a large egg. Also, the margarine may have been too hard. "
I think it explains why your cookie is too crumbly.hope this helps