To quote the judge at my local county fair (ouch I hear this every year) ... inconsistency in the singles will result in drifting during the plying. Which is to say (and I apologize for sounding pedantic, I can't think of a funny way to put this) ... that if the amount of twist in the singles is variable, then your plying will be difficult; and a low twist area in a single is very likely to drift apart during the plying.
If there was alot of drifting apart of the singles, then likely they were too low twist for plying in general. When I'm spinning the singles, I'll use a ply back test so I can see what my two ply would look like. I'll go even further and save a sample of the ply-back so I know how much twist to put in when I ply. If you test the singles regularly (every length, every third length, whatever works for you), then you'll get a feel for how much twist to put in to keep the singles consistent, and ply-able (ah a pun! LOL)
I recently re-read Paula Simmon's Spinning for Softness and Speed. Her drift (it must be Punday...) is that you will get consistent twist in your singles if you use point-of-twist drafting, feeling the drafting amount in the palm of your hand. It's really quite cool, if I do say so myself, to actually feel this. Paula also says it's one of the fastest ways to spin. Having filled up a bobbin of about 12-WPI singles in no time flat with some Tunis roving from my stash, I do agree :-) But I'll also agree with her, that applying it to fine spinning takes time! My finely spun fibers want more twist than this method naturally puts in, at least for the tight ply I'm shooting for, and for the level of skill the palm of my hand currently has for this method. I'm sure with practice it will learn!
* How do I make sure my singles aren't underspun?
* How do you organize your spinning? (sample cards)
* How do you make a good looking two-ply yarn?
* What does it take to win a yarn competition?
Questions? comments? Post them on this blog or contact me. Thanks!