Check for any out-of-sequence (OOS) event and correct logic if necessary. Wrong logic can result from both, retained logic as well as from progress override. Retained logic keeps the precedence sequence at the expense of the remaining duration to happen 100% after the predecessor finish. If the predecessor finish is way into the future then remaining duration will start way into the future and if remaining duration is large then the activity finish will be finished way into the future + large remaining duration.

Out of sequence is not that common but do happens, eventually you will have to deal with it in the few occasions it happens. Get used to check for OOS after every update.

With the project open, go to View > Filters

There should be a default filter called "Critical". -Check that box and hit "apply"

If you can't find the default filter, you can create a new one by hitting the "new" box, then making there rule "where Total float is less than or equals 0"

Do you have a copy of the programme from before you updated it? If so, you can set this as the baseline for the updated project, and track the variance in dates between each activity before and after updating

You cna of course also just look at the critical path of the project -this will tell you which activities are driving the end date.

If the project has slipped by a month after you have completed a monthly update, this suggests that one or more activities which were critical last month made no progress at all/ -That or they were mistakenly omited from the progressing.