A previous client of mine has this problem. HAR listed 2 full baths and their home had 3 full baths that were original with the build of the home. When discrepancies of structure are involved, these have to be addressed with HCAD, then the agent can update on HAR.
If previous sale is not listed in HAR history there are a few possible reasons. The sale may have been completed outside of the MLS system like a for sale by owner, foreclosure etc. HAR will only keep records of homes sold within the system. If you are a member of HAR you can look for history that may not be available to someone outside the system. Tax profile history may also show previous sale dates or you can pull records from the County clerk to see when the deed was transferred. The other reason may be time, HAR doesn't keep all history, so you may not see sales history for a home that was last sold 15-20+ years ago.