There could be a number of reasons why WhoKnows is not recommending a colleague when you're doing a search.
The most likely is that nobody within your network has expertise in that topic yet or has contacts that match the search criteria.
WhoKnows does not recommend colleagues to help until we're certain you're looking for help, so recommendations are not always shown on your first search in a topic area. You can force recommendations by putting quotes around your topics. This will do a direct match against your colleagues' profiles and find matches.