I imported from last.fm (which has ~55k scrobbles).  After the first import, only ~39k listens were shown.  After doing a second import, ~49k listens were shown.  No errors were shown during either import.

More details sequence of events:

• I have an existing last.fm account https://www.last.fm/user/avh4 with ~55k scrobbles
• I created a new ListenBrainz account https://listenbrainz.org/user/avh4/
• I ran the last.fm import, which succeeded with no errors
• My ListenBrains dashboard showed only ~39k listens (missing ~16k listens)
• I did a ListenBrainz export, and the downloaded JSON matched that (~39k listens)
• The next day: ListenBrainz stil shows only ~39k listens
• I ran a second last.fm import, which succeeded with no errors, and I specifically noted it said "Successfully submitted 55383 listens to ListenBrains"
• My ListenBrains now shows only 49,810 listens (still missing ~6k listens)

I think it's possibly my last.fm history might have some duplicate entries in it that might account for some of the missing listens, but I think that would only be a few hundred listens, and not thousands of them.

I have the listenbrainz exports saved from after each of the imports, so lmk if it's helpful for me to try to compare them with each other or with my last.fm history.

I also plan to try a third last.fm import tomorrow to see if that brings in more of the scrobbles.