What path payment are you trying to find?
In order for Horizon to find a path, there needs to be a path of outstanding orders that can be taken to go from asset A to B that complies with the requirements of your path (maximums and minimums defined).
For example if you want to send 100 B using a max of 300 A, then if there's an open order for 600 B at an exchange rate of 2:1 (A:B) you should be getting a path.