There are definitely problems with filtering with some types of data source.
(whcta are you using?) If you click the drop-down at the top of one of the
columns in the Mail Merge Recipients dialog box and select Advanced, you
/may/ be able to see what Word is doing - sometimes, for example, it
duplicates the last condition in the list that you can enter there.

However, Word complicates things by using two different methods to filter.
If for example you use the drop-down to select a particular value (say,
Country="UK"), Word generates a bit of SQL like "WHERE (Country = "UK" )"
and uses that when it gets the data from the source. But if you
check/uncheck individual entries, Word just sets a flag internally for each
record selected/unselected and remembers to include/exclude it in the merge.

I'm not sure that that helps you, but if you are using the second method, it
might be worth seeing if using the first method instead works better.
Otherwise, re-connecting to the data source is actually likely to be the
simplest way to clear out the old filter conditions. However, if you are
always using the same source, it may make sense to record a VBA Macro that
closes the source and re-opens it, and assign it to a toolbar button. That
way, you should be able to restart with a single click.

Peter Jamieson