I have 3 in the 600 touch and sew series. If you start working with the bottom tension, it's the tiny screw on the silver bobbin holder. I'm not home for a few weeks, so I'm trying to remember how this looks. I believe it's on the right side. I've adjusted mine a couple of times (different machines). My technique is moving it to the right a quarter turn. Try it out to see if there is a change. Turn it to the right again a quarter turn. Is it better or worse? If worse, return it to the first position and turn it to the left a quarter turn. Whichever way, make slow adjustments until it is corrected.
However, if this is your first Singer slant needle, before you start on the bobbin tension, check the book to make sure the bobbin is feeding the right direction. I've found it feeds the opposite direction than my Janome 6500 and several Brothers that I picked up. Lastly, make sure there is no thread or fuzzy stuff in the bobbin area. Then try the tension adjustment.
My daughter messed up my 600 when she lost her temper learning to sew. The thread knotted and she yanked rather than unhook or cut the thread. I replaced the bobbin holder and hook. It worked fine after that. The new holder has a touch of white paint on top of the screw to keep it in place. If you get it adjusted right and it slips again, you might try a drop of paint or even a bit of nail polish to keep it where it needs to be.
They are neat machines but do like a gentle touch ( no pun intended).