When making tubular nets I find it better to use a larks-head onto a hoop. I also cast-on what looks to be way too many loops so the net has the ability to swell below the hoop - or - to replace the hoop with something else that opens the mouth of the net to a larger opening and restrict it further down the net by stitching in a smaller circle of cord. If you use larks-heads, they will untwist and straighten out if you remove them from the hoop. Some books I have seen, say to cast-on with a clove hitch which will not untwist if you want to remove the hoop or extend the net by adding cord to the "top" edge.
As for slipping knots, what material are you using, man-made fibres tend to slip and so require a double knot, whereas natural fibres tend to bite together better and so you can use a single sheet-bend - that said, some slipping is almost normal - especially if you get into a fast rhythm and sacrifice a little quality for speed!
ATB
Ogri the trog