tbh i'm not sure if bolton and mulvaney were subpoenaed or not. i know the white house ignored several subpoenas served by the house.
what the house should have done was go to court to try to enforce the subpoenas, but i'm sure they wanted this all wrapped up by the election and going to court could have taken years.
the problem with the dems' approach is what we're seeing now. the republicans control the senate and the democrats should have realized there was a chance the senate would just refuse to call anyone.
note that this is entirely semantics as trump could admit to everything the democrats are alleging and he would have no chance of being convicted.