OK, heres my ideas, take them for what is worth. I see 3 things that might be causing you trouble.
Fuel, how much fuel do you have on board when you try this? A heavy 38 is a slow 38. Its a much different animal if your fuel heavy. It can be flown, but when your slow it is even harder to get her to roll over.
Combat Trim, combat trim in the 38 can make that plane do all kinds of odd things at slow speeds, or high AoA. For most of my flying I leave CT on, but when I'm going to be throwing the plane around or doing stall moves I flick it off.
Rudder, you said you used it, but not full. CT may be giving you trouble, but I've never had any trouble giving it full rudder. Full rudder is very important to get her to roll over. Check out the BFM film posted of Rolex's recent class. In the first section they do some Split S's and he shows the difference in roll rates with and without rudder input. Its even noticeable in the pony they are flying.
So any one of the things can give you a nose up condition, even not getting your roll all the way over.