When I ran it for the first time (1 loop) last Tuesday I felt like a demigod and was singing to myself for a while. Took two days off and started listening to it again last Friday (1 loop/day), it’s been three days now but the results seem to have slacken a lot and only after using Rebirth Ultima I can see it back… but still not in its full power.
Any ideas? Is it normal and I should keep going and see how it goes?
Apart from it I use Rebirth Ultima (1 loop/day, no day off).
Yes. After an initial dramatic flare, the program is now being internalized into your mental structure. It’s becoming a natural part of you.
Same thing happens when you first start working out. You’ll see a big improvement right at the beginning, and then as your body adapts (and begins to develop into real strength) it will seem less subjectively dramatic.
This is a good thing. Possibly indicates that this program is ‘compatible’ with your mind.
What is appropriate now is to make plans that use the Stark capabilities. USE IT.
Think of it this way: If we were always blown into an amazed stupor by January, we’d never get to March.
You’ve gotten used to January. Now march your ass over to March. Progress. 
Don’t chase the high. Like @Malkuth said, when you first run a sub, you’ll sometimes feel on top of the world, then as the subliminal scripting becomes normalized, you won’t feel those acute effects. That does not mean the sub isn’t working. A lot of people make the mistake of quitting at this point. Keep going.
The Ultima Core has a lot of “feel good” scripting in regards to productivity and taking action, hence why it comes back.