The phrase "a bit frustrating when" is correct and usable in written English.
It can be used to express mild annoyance or disappointment regarding a specific situation or event.
Example: "It's a bit frustrating when the internet goes down right before an important meeting."
Alternatives: "somewhat annoying when" or "a little exasperating when".
He values loyalty and a conservative approach, which can be a bit frustrating when Manager Dusty Baker orders another year's worth of bunts.
It's awe-inspiring – if a bit frustrating, when you need to concentrate – to find such apparently recerche rooms as the gallery of pre-classical Greek pottery constantly crowded with people of all ages.
Improvements in spear size and expansions of seasons have led to some varieties being less hardy, so a harsh frost would knock things on the head (a bit frustrating when you've waited so long for those precious pickings).
"Sometimes it's a bit frustrating when I can't tweet what I want when I want to banter with my friends.
"It's a bit frustrating when we don't have really good earthquake locations," Michael said.
I guess it's a little bit frustrating when you know what a guy's going to do and he still gets you out.
This can be a bit frustrating at times when something out of your hands goes wrong, but that's true for any coaching job.
For instance, the poems -- though superb and relevant to the story line -- can be a bit frustrating to get through when one wants to return to the absorbing narrative.
"If we're going through this process, which, yes, can at times be a bit frustrating even if it is necessary, when somebody fails a test, don't just let them back into the sport 18 months earlier than they should be.
I am sharing my room with Camille Muffat and that was a bit frustrating because every night she was asking me when I would get my medal, so today I finally get it.
The remote-control lighting system was a bit frustrating, but I blame myself for not paying attention when the bellhop explained it.
