One common problem
The most common reason I've encountered for this kind of problem is that the music being created is too complex to remember.
When improvising, there are so many details to keep track of that our memory easily gets overwhelmed when we want to return to an earlier part of the music or create a new one.
A good exercise
The remedy is to practice very simple phrases that are remembered with little to no effort. This might mean only one or two notes, if that's what it takes. And the simplicity applies both to the notes and the rhythms. But by using these easily remembered musical materials, one can practice the act of memory while improvising.
- Improvise an easily remembered musical gesture.
- Improvise a second easily remembered musical gesture.
- Replay the first gesture.
- Add a third.
- Replay the first and second.
Another good exercise
Another improvisation/memory exercise I like is to:
- Create a brief melody.
- Replay the melody, then extend it with something new.
- Replay only the new part, then extend that with something new.
- Replay only the most recent new part, then extend it with something new.