Bad Results after washing

We have had many issues with shirts after their first wash. We have used recommended settings from this is site.  The transfers heat press on just fine, we have tried the top end of the recommended temp setting as well as lower.  We have the finishing sheet and it does not make the shirts last in the wash. Cant figure it out, are we making a mistake or is this process just not going to produce shirts that have any longevity to them?  Thanks for the help!