McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam, McFlyFoam.
The stuff practally ties itself when it comes to making nice round eggs.
Glo-bug yarn or similiar products do not.
The McFLyFoam, a synthetic yarn despite it's name, has a spongy feel and the tiny indiviual fibers are very "kinky." The yarn also has a velco like quality when you handle it with dry fingers. It compacts and flares a lot when you put a tight wrap of thread around it.
GloBug yarn on the other hand is very soft, and has a whispy quality. It's great yarn for suckerspawn style eggs. I think this material tends to have more natural color and translucency than McFlyFoam but it does not tie very good single eggs. It takes a lot of fiddling to get it to flare around the hook and the spot where you wrap it with threat tends to sho in the finished fly. But again, it's fantastic for suckerspawn style flies.
If you have McFlyFoam use a sturdy thread like Uni in 6/0 or UTC and try to use a minimal number of tight wraps in a compact area to secure your yarn. Also use very sharp scissors.
If you have some other yarn, consider tying suckerspawn style. You may need to pull your strand of yarn in half so that you have two or even three thinner strands to work with. This will give you the appropriate density of yarn to tie a good looking fly.
FWIW, I prefer the suckererspawn tied with GloBug over single eggs tied with McFlyFoam. So give them a shot if the single eggs aren't working with your yarn.
Kev