The Ellisimo has auto tension and seems to do a good job so doubt it is the tension. Did you rethread the bobbin as well? Also make sure that the thread isn't getting caught on the spool if you are using one that has the little slot in the top of the spool. Another option is to turn the machine off and let it reset everything to default.
Good luck and let us know when you find out what the problem is. (I have an Ellisimo so will be watching!)