Log into your WHM. In the URL below change "yoursite.com" (twice) with the site you use to log into WHM.
If it gives you an error then we know why it doesn't automatically create the accounts.
Code:
http://www.yoursite.com:2086/scripts/wwwacct?sign=&plan=undefined&domain=aaa.yoursite.com&username=aaaa&password=tester"a=10&cgi=1&frontpage=1&maxftp=0&maxpop=0&maxlst=0&maxsql=0&maxsub=0&maxpark=0&maxaddon=0&bwlimit=50&cpmod=x2&customip=--Auto+Assign--&contactemail=test@test.com
Edit: BTW I have WHM version 10.8.0