It may be that all of the units are in use on all of the virtual cards you have defined so far.
If this is the case, simply programming a new unit on an unused card will allow you to use the LUU or LUVU commands to generate good results.
Remember, you can use virtual cards 0 - 15 in your virtual loops, so for instance if you are currently using cards 0 - 10, you can just program a new unit on <loop> <shelf> 11 0, then when you issue the LUVU command against your loop, all of the remaining units on card 11 (1 - 31) would show up in the output.