I am with Sam on this one. It sounds like you are overtraining by lifting too often. I also only train every fifth day, and progress keeps coming. But you may also just need a break as well.

I find that after about 12 weeks I simply cannot progress so take a fortnight off.

I would add in an extra rest day after each workout and see if your lifts improve. If not add in an extra day. Some people are real hardgainers and can't progress strength wise if they lift for a bodypart more than once every 7 to 10 days.

Everyone is different.

You may also find that some bodyparts can take more exercise. If so then you could trian them on days that are not main workout sessions, so you still feel like you are active. I think I could train my forearms every day if I wanted, they seem to react totally differently to the rest of me.

But the issue may well be nutrition as well. You need carbs after resistance training if you want to make decent progress in terms of size and strength. If you are not getting that then it may be it is the cause behind lack of progress.