Hi Barry,
Glad you like the book. Most of the difficulties you mention seem to relate to the age-old problem of "crowding the head" of the fly. I sympathize, as I'm sure most tiers do; nearly all "new" tiers confront this issue as they attempt to refine their skills.
Here are some ideas that helped me to overcome this issue in my own tying:
Plan for "headspace." Early in one's tying experience, materials seem to have a mind of their own and are constantly migrating toward the eye of the hook. Rolling, slipping, using too much material or thread, or inaccurate placement all tend to produce this frustrating effect. If a thread wrap or material ends up somewhere other than where you intended, unwrap and try again. If you get the same result, try to figure out why it is happening. Some of our most valuable tying discoveries occur during this (admittedly frustrating) trial-and-error process.
Practice "economy" in the application of thread and material. Never use two wraps if one will do. Every wrap of thread should serve a purpose, and random or misplaced wraps are the enemies of economy and accuracy. Keeping the tip of the bobbin close to the fly is a tremendous aid toward accurate thread placement. At each stage of construction, consider the current purpose of the wraps you are applying and make sure those wraps accomplish that task. Two appropriately tensioned and accurately placed wraps are always better than ten random and inaccurate wraps.
It is usually easier to add material than to remove it (or worse, to try to "work around" too much material). Controlling a "noodle" of dubbing can be tricky. First, a long noodle tends to defeat accuracy by keeping the bobbin tip too far from the hook. Second, long or carelessly twisted noodles tend to expand and unravel as the wraps are applied. (I twist dubbing onto the thread in one direction--the direction of the thread's twist. Try not to roll back and forth, and avoid snapping your fingers at the end of a rolling stroke. Both of these actions tend to loosen dubbing.) While most experienced tiers can efficiently wrap an abdomen or thorax in a single dubbing operation, there is nothing wrong with applying multiple thinner or shorter applications until this skill is acquired.
As to the specific problem with the poly-yarn wingcase, I suspect you may be trying to use too much poly. A moderate hump is the goal, but "big" and "puffy" may be overkill. The forward rolling or slipping of the thread as you attempt to secure the poly at the front of the thorax can also be caused by a bulky overdubbed thorax. Unlike nymphs where the thorax dubbing is to be picked out to suggest legs, thin dubbing is more effective, especially over weight. (Thick dubbing creates bulk and can even defeat weight and balance.) I like to secure the poly with a few tight wraps that seat just into the forward edge of the thorax dubbing rather than onto the hook shank. That way the cut ends of the poly can be wrapped down in the headspace without the securing wraps slipping forward. If you have trouble dubbing the head because of a severe taper or ramp, try "reverse dubbing" (winding the thread to the eye and dubbing back to the securing wraps with very thin, tight dubbing). You can then make your whip finish or knot at the front of the thorax rather than the eye of the hook. This is not traditional, but I do it whenever it seems easier or more appropriate and find no disadvantage.
Make sure you can see what you are doing. Use good lighting and background, and check the "dark side of the moon" (the other side of the fly) frequently. I have a fishing friend who claims that one of my most valuable assets as a tier is excellent close-in vision. I am slowly losing this edge with age and find that I have to pay more attention to things that make it easier to see. (Though I haven't resorted to reading glasses--yet!)
Proportion, accuracy, and economy are hallmarks of accomplished tiers, but these are hard-won skills. Even when tying flies of my own design, I still get frustrated when my skills lapse into sloppiness. Some days my tying is easy and effortless; on others it seems clumsy and accident prone. But a well-tied fly is always worth the effort. (Despite this, many trout have succumbed to less-than-stellar tying skills in the past and will continue to do so.) Best wishes and good luck!