Sorry, David, for not being specific enough.
Simply put:
1. I downloaded the recent HU pack (3.5) for a review.
2. Found some strings with typos, etc.
3. When I entered a (sub)string awaiting correction, Amos appeared to find only a portion of such (sub)strings.
Other settings do not appear relevant, as they are the same in both cases (in case of finding and in case of failing to find a substring).
Puzzling...