while tying flies that call for a thread built head, I'm having trouble keeping the larger back part of the head from pushing the smaller forward part of the head over the eye of the hook. What exactly and I doing wrong?

Gib, on large hooks I "rough up" the hook shank with a file and super glue thread base as I build up the head.
Hope this helps.

good too know!

Depends on what size thread you are using. Could be too much thread buildup, not enough thread tension, or both. Try putting more tension on the thread and don't build up a ton of thread at the hook eye, just enough to make the head'll do.

Keep it tight! Try to wrap with as much tension as the thread will allow before breaking. If the heads look sloppy or loose, switch to a smaller denier thread.

I sometimes still have this problem. What I try to do is build the thread base proportional. If you do all the back, then work your way to the eye, you will have slippage. Work the thread on front and back. Then add a bit more to the back, but still go forward with it some. Keep your thread tight, not loose. Make sure when you are going to tie your head, that you left enough room in the fly, to build a proper head. Some people prefer small heads on flies. I think it depends on the fly. I tend to make larger heads on my flies, than a lot of people do. I always finish with head cement. Practice makes perfect. Concentrate on how much space you need to leave for your head. You will get an eye for how much room you need with practice. Salmon tyers use thread wax to build heads. I don't think most of the time it is necessary. You also may consider going to a smaller thread. I like 6/0 Flymaster Plus thread for most of my tying now. I used to break thread constantly, even on the larger flies. But now I am getting better at it, but I still break thread now and then. I also have smaller threads, and I am getting better at using them, but once again...it takes practice, and muscle memory. I hope this helps.

thanks for all the suggestions. I think one problem is i dont put enough tension even though its tight. I have a medal bobbin. I plan on upgrading my vice that is slowly breaking and my metal bobbin that breaks all my thread too ceramic

Get stronger thread, more tension more better and use superglue with each new layer. Thread can only do so much so if you want a pronounced head use a bead.

Try running a few tiny beads of Zap-a-Gap down the thread (between the bobbin and hook) before you wrap it. Not only will it help keep your thread wraps in place on a tapered head, but it can also be used to help secure other materials that tend to slip out from under tight thread wraps - like bucktail.