Is this the longarm that is timed to a specific needle size? (size 18, if I remember correctly?)
If so, you will have to use pretty much *only* that size needle and that means that you will be able to use only thread that works with that size of needle.
If the thread is too fine or too coarse for that particular needle, your machine may not be able to form consistent stitches or even any stitches at all.
So, if it's timed to a size 18 needle, you are going to need to try some pretty heavy thread - size 30 or 40 - in the needle. See if that works better and we'll try to figure it out. :)
If this isn't the machine that's timed to that one needle, then let's go step by step to troubleshoot.
What size needle do you have in the machine and what size thread are you using in the needle?
Does your machine have the ability to sew using feed dogs to guide the fabric or is it strictly free-motion?
Are you practicing with a sandwich of fabrics with batting between them or just a piece of fabric?
I'll bet we can figure this out. :)