First of all, you need to analyze: What is the height difference between your solar water heater and your shower head? See if the pressure is reached?
Then check if there is scale on the solar water heater. The pipe is blocked.
If the pressure is also reached, there will be no scale. Just add a booster pump and that's it.
Possible problems are probably:
1. Raw material tape blocks the pipe connection or debris blocks the water outlet.
2. A main water supply pipe has too many branches or the water supply pipe is small.
3. Check whether the solar exhaust hole is blocked, etc. It is recommended to check the pipeline first. If other problems are eliminated, it is recommended to install a small booster pump.