Select song & press enqueue button. Try closing LW & re-opening. It might be due to lag or something.

If all that fails, make a note of all your settings under Tools>Options ... and then delete your LW Preferences folder whilst LW is closed; instructions here: Point #2 & 3 in Fixes for Limewire (sample image included at bottom of page.) But be sure you use all the same settings because once you restart LW, you will need to set up all your options again just as you originally did when you installed LW.