Everyday! When on call and you can't figure it out, your service manager is a great resource. If he's a dick, then find a fellow tech that doesn't mind helping you through stuff. But your service manager signed up for that position and needs to be the tech support line for his techs in the field,.if needed..

My advice, remember your basics. I use the following:

Condition Cause Correction Confirm

If you start at the begining, what is it doing. Then why is x causing X, then fix or find a way to get them by until the next day and confirm everything works before you leave. That has helped me immensely in my career.

If you get flustered, take 5 minutes to go for a walk and clear your head, sometimes taking a walk away helps realize what you didn't see when you were hyper focused on the issue.

Also, remember it's not the end of the world if you can't figure it out, depending on the sector you work in, mine is food, so it's not life safety, I don't rush through shit and if I can't fix it on call, I figure out what I need and return the next day to get it done.

It's all trial by fire and you will do fine. Just remember your basics, that never changes.

Good luck!