Hey guys -

I went out to fly and my quad wasn't arming. Then all of a sudden it armed and motors 1, 2, & 3 would spin, but 4 had no response.

I got home and tried it again to look at the arming flags on Betaflight, the thing worked totally fine. No flags. Armed with all 4 motors.

Took it to a field today and when I tried to arm, it didn't work again

Betaflight arming flag: RPMFILTER

But sometimes the quad works just fine?

Any ideas what issue could be causing this?

Its a writing issue somewhere.

I have no clue what stack or AIO you are using. But if its a stack then check the wiring harness and pins (visually and using a multimeter).