4 scrimmage ads ... are just not enough
Those ads change at every refresh, and you have a maximum number of challenges you can issue (15, I think). Therefore, after 4 or 5 refreshes of the Manage My Team page you should be all set. Takes a couple of minutes.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."