How often do you find yourself working on a particularly tricky bass part only to get frustrated and walk away from it.......only to try it again weeks/months later and it comes to you easily?
What's that all about?
This is normal IMHO. I have experienced it many times.
I think we develop mental blocks when we are working on difficult tasks. We may actually learn to do a task, but the mental block prevents us from actually doing it. Take a break and your brain keeps working to master the task, and also unlocks.
When I prepare for a tour, I go through the music and identify all of the sections I need to work on. When I start to get frustrated with a section, I put it down and start working on the next session. I keep doing this until I have worked through all sections. Then I start over again. The idea here is to avoid pounding for so long that I become frustrated and blocked.
Before working sections up, the first step is to work out fingerings. After I finalize the fingering, each time I work through a section, I slowly increase the tempo until I hit the wall and can't play any faster. Usually each section will get a little faster and cleaner each time I cycle through. But the next time I come to the section, I will slow it down a bit, and then work it up to a slightly faster tempo.
When I feel I have essentially mastered a section at a tempo that is a bit faster than it will be performed, I set it aside and continue cycling through the remaining sections I have not mastered yet. Eventually I reach a point where I have either mastered all sections, or I need to let it go for the day.
The next time I practice, I review the sections to determine which are good to go and which need additional work. Then I use the same process. Usually progress goes much faster on succeeding practice sessions.
Some times I get stuck and can't master certain sessions by performance day. Sometimes you have to simplify, and play an easier line that serves the music.
Frequently the music is turned in a few weeks after the last performance. So there is an extended break where I have not played or worked on the music. If I try to play through the sections I failed to master, I have often noticed that my mental block is gone and I can actually work the section up fairly easily.