Peaking quickly, then declining

To some extent, this is a natural response of our nervous systems, I believe it is called accommodation. The nerves get used to the new stimulus and at some point quit responding to it.
More importantly, in your case I suspect that the masturbation and traditional orgasm, while enjoyable, are short-circuiting the rewiring process. There have been many posts about this in the past. I think at this time you should avoid any penis contact during your sessions, and avoid going for an orgasm, see if that makes a difference. Better to learn to nurture and prolong those initial sensations, the orgasm can wait.
I have experienced the same thing (goes back to sleep after 20 minutes). Sometimes I was able to reawaken it just through meditation, other times I would take it out an try again in an hour. As time went on, it has become easier to reawaken it, but it is never a sure thing.

what you describe has been typical of my aneros journey since very early on. THis has continued for almost four years, however, with the time, I have found that my peaks last longer and while the sensations do calm down, it takes much longer. I have always used several models during a single session, swaping out for a different model as the sensations die down.
Although this has taken ma a long time, I now don't really have that conditon. I now can have wave after wave with the same model and while I still do change out during a session, I might go for an hour without changing.
My advice, like Woodsmans, would be to avoid any penile contact and stop masterbating as a part of your sessions. Give it time and enjoy whatever you get out of it.