What others have posted reminds me of another problem I had - I was winding my bobbins way to tight. The bailey was a class 15 machine so I used my old Singer 15 to wind bobbins, and would hold the thread between two fingers to help guide to so I a had a nice evenly wound bobbin.
The fusion came with a winder, and I cranked the tension a bit and with also using cotton thread, I was actually stretching the thread on the bobbin - it caused me to have very inconsistant tension.
I am also very diligent on cleaning lint out of the bobbin shuttle area and the bobbin case, including under the tension spring. I've had lint build up behind the bobbin race (where you couldn't see it) and it started forcing the bobbin case a bit forward and that started causing top tension issues as the thread was getting hung up a bit as it slid over the bobbin case. I have a small vacuum cleaner with a narrow hose end to suck lint out of the machine. I was told not to use compressed air as too often it can blow stuff further in - but that is for my specific machine, HQ recommends using it on the Infinity.