I actually had the opposite problem a few days ago (auto-completion and doc-popups not working, but error highlighting was). Everything is working now though, and I didn't try to do any troubleshooting at the time, so I have no idea what fixed it (possibly a system reboot, but I can't say for sure).
For reference, my VS Code is the latest version (1.22.2), and I'm using Nim 0.18.0.
@nucky9: same versions of VS-Code and Nim here.
@honhon: checked for leftover/crashed nimsuggest instances, none found. Vs-Code's Nim plugin starts multiple nimsuggest instances, one for every
EDIT:
folder (Nim package) in the workspace. <-- WRONG
source file for which nimsuggest functions are used. <-- CORRECT
The problem remains even after a fresh install of Nim and VS-Code.
Narrowed it down: code checking only works for the first folder in the workspace. So my on-the-fly "fix" is to drag the folder to the top position in the explorer panel...