[Pisces]

Once you assigned a tracker on broadband, you cannot assign a tracker to that contact on a specific frequency (wavy line) in narrowband. You need to free the tracker in broadband first. And vice-versa if you want Demon data (not available from narrowband trackers in vanilla DW at least), you need to unbind the narrowband tracker and assign a broadband tracker to the contact.

The assign button also does not work if there are signals of 2 or more contacts overlapping. In narrowband you may be able to assign the tracker to a unique frequency. Slightly shifting the bearing marker at the top of the bearingscale should show brightness changes in the frequency lines (they change brightness in sync) to indicate which frequencies belong together. Choose the brightest one that isn't overlapping with another frequency.

Also your classification is done on the marker on the map. Right-click to open the classify menus. The filter on the sonar interface only shows which classes fit the lines in varying degrees of likeness. The more lines of a specific class it can show, the more likely it filters out the only one possible remaining.
It doesn't make the classification stick to a contact. (though Sonar autocrew may classify for you and make it stick)