Not compiling may be due to being out of memory on the micro:bit. It’s possible that if you’re on a V1 moving to a V2 will help. I think Frank’s example of compilation breaking after adding in just one more feature is likely due to this.
Extensions can use a lot of memory and, in some cases, can be “trimmed” a bit. For example, in response to an earlier thread of conversation about the MBit app, I made a simplified MaQueen library that omits some features. (Here’s the discussion: Android app/Discord - #13 by bsiever ; Here’s the URL for the condensed MaQueen extension — it can be pasted in when searching for an extension: https://github.com/bsiever/pxt-maqueen)
Bill