In Setup > Rules, click on the tab in the column at the left titled "Special Recognition".
Scroll down to the area under the blue bar "Individual Special Recognition".
Look for "Rules for individual most improved average recognition:",
and look for "Use bowler's entering or Book Average instead of designated number of games:"
and set that switch to "NO".
The next item is "Use bowler's average at the following number of games:"
set that item to "21".
You will need to go back to before week 7 (before anyone reached the 21-game mark) and then advance back through to the end of the season or current week to get it to process correctly.
Above the above blue bar is a setting for "Number of games bowled to be eligible for any league recognition at that time:" You will have to set that according to your league rules; most specify 2/3 of the season. You will have to determine what the minimum number has to be (63 if a 96-game season, for example, if 2/3).
To see who won, add the item to your standings sheet. Note that if you run the report in Reports > Most Improved Average, that will include everyone alphabetically, but the standings will show the top improvers only.
Always check, download, and install the most recent update of your software to ensure that issues may have been corrected.