Mac won't let me startup from external SSD

[Lior]

I have an External Ssd with Catalina Os.
for some time every thing was ok and my Mac was working on my SSD and even restarting from it as it should.
now when I try to restart from the External SSD I go to sistem preference and under the startup disk tab I choose the external SSD and I can see the it say that it would start up from the external SSD but still it startup from my old built in hard drive .
does anyone has any idea how to fix it ?
(sorry if I'm spelling wrong or if I'm not very clear English is not my first language)

 

[Howard Brazee]

I don't know the answer, but the first thing I'd try is to run Disk Utility/First Aid on the SSD. It can't hurt. Another thing to try is changing the cable.

 

[Harry Trent]

I would do a fresh install on the external. Sounds like you may have some corrupted boot files on it. Of course, back up your data first if there are things you don't want to lose.