Probably what you are experiencing is a known phenomenon based on "blocked practice" vs. "random practice." Blocked practice is when we do numerous practice runs in a row. So...if you're practicing a double bass pattern, and you repeatedly do the pattern 50 times, that would be blocked practice. Random practice would mean doing the pattern 50 times as well, but not all in a row. With random practice, you might do the pattern once and then spend 15 minutes working on reading or hand technique. Then do the pattern once again, and take a break to eat lunch or answer an email. You would keep this up until you eventually reached 50 repetitions.
Scientists have been studying the effects of these 2 approaches for many years, and the results have been extremely interesting. When people use blocked practice, they achieve much greater results during the practice session itself. So...using our example from above...the 50th practice run of a person who did blocked practice would be much better than the 50th run of the person who did random practice. HOWEVER...if both of these people stop drumming for a couple of days and come back for what is called a "retention test," the findings are startling. The person who did the random practice will wipe out the person who did the blocked practice. Incredible, right? The person who did blocked practice will find that they cannot get close to what they achieved on their 50th run during the practice session. Meanwhile, the random practicer will find that they can perform at almost the same level as where they left off during the practice session.
I think you are doing some version of "blocked practice," which leads you to a deceptively high performance level during the practice session itself. Then a few days later, you are coming back to it and administering a "retention test" to yourself. You are finding that your performance is far below the level where you left it. It is the same thing that scientists have been seeing for many years when studying this type of thing. The solution? Now that you know about this, start using random practice.
Best of luck.