I am currently working on a pair of socks that has a purl at the beginning of each pattern section. I couldn't get it to look tight enough with magic loop (I don't like using ml anyway haha) so I restarted on two circulars and the first sock came out perfect.
What I did was to pull the first purl on the row really tight. Also, it
looked like there were ladders until I had knit for several rows and I
just gave a little tug down on my knitting and it all evened out. I would
say just make the first purl tight and check for ladders in the knitting
that is several rows away from the needle to get a more accurate
view of things!