Why does my shower do this?!

[elephantcakewalk]

When I turn on my shower it dribbled out unless I open the bathroom sink tap Once I've opened the sink tap it's starts working properly. Any ideas?! Does it need fixing?

Possibly an air lock in the system

Thanks is that something serious or not really? Ie can I simply just turn the bathroom tap on every time to use the shower or will that cause further problems?

It’s one for @PigletJohn.

I think it’s easy to fix but you need to follow a certain process.
Which PJ will know.

Do you have a combi boiler?

And a thermostatic shower mixer? How old?

Does the bathtap work fine?

Photo of the shower may help.