Ditto on all the above suggestions. Re-thread, larger needle, check for burrs, hold the thread ends when starting.
Have you considered getting a teflon roller foot to improve fabric handling since you say your fabric is slippery? My Hubby sews canvas a lot (for his black powder reenactor hobby) and uses a roller foot and size 18 needles for the heaviest work. The 15-91 uses low shank attachments so a aftermarket roller foot shold be easy to find ... You say you recently got the machine - did you give it a good oiling everywhere before you started using it?