If I've read butcher's post correctly, he is only doing 1 set for bb curls and hammer curls each session. He's also doing 2 sets of over-head tri-extensions each session. Assuming 3 x weekly sessions that's 6 sets for bis and 6 sets for tris a week. Guessing at what he's been doing before I think that's okay.
What I want to know from butcher is:
1) Have you completed a cycle or is your observation based on the first few weeks of a cycle?
2) When you were doing Max-OT were you doing 6 to 9 total heavy sets per muscle group and doing 4 to 6 reps per set?
3) Were you training arms once a week?
4) How many total reps are you doing for bis and tris during each phase of your HST cycle?
5) What sort of loads are you using for bb curls, what size are your upper arms right now and what is your level of body fat?
If you are only doing six total sets a week during 5s then I think that's a likely reason why you may not get the arm growth you are after. Here's the thing: if my understanding of Max-OT is correct, you were doing around 30-50 total reps for bis (and the same for tris) with a heavy load all in one session. That's going to pretty much ensure that you get a decent PS response from your arm work (it's probably more work than needed to be done, but at least it's not too little). Then, during your back, chest and shoulder sessions you are still stressing your bis and tris so you are still training them with a fairly high frequency.
I think what's probably happening, now that you have switched to an HST workout schedule, is that you just aren't doing enough direct arm work each session during the heavy part of the cycle. If you are only doing two sets of 5 reps each session then, while that's a total of 30 reps a week, it's only 10 reps a session. It seems that that is not enough for you to elicit a strong enough growth response with your present level of conditioning. Try 3 sets of 5 reps for bis and tris each session and see how that works out. If your arms are very conditioned to using heavy loads then you might well need to push the total up to 20 reps per session.
Hope that makes some sense.
If you haven't even reached the 5s yet, well...