If you were using a bacterial culture to grow and count viruses, and
you ended up with no plaques on your plate, what might an explanation
be?

Acually there are two type of bacteriophage (virus surviving and
replicating in bacterial host).
 First type include LYTIC PHAGES: these are the virus which cause the
lysis or death of cell and released in outside environment. In these
kind,  phages DNA or RNA  replication is immediately followed by
synthesis of capsid proteins and these are assembled togethor
resulting in the formation of complete phage particle which is
released after causing lysis of bacterial cell.
 
Second types of phages are LYSOGENIC phage: these are the phages which
can integrate the genetic material (DNA) with the host bacterial DNA
and replicate generation after generation in this form only. When the
bacterial cell divide phage genetic material can also be transferred
to daughter cells in integrated form. This can continue for several
generations without any harm (lysis) to bacterial cell. One example of
these kind of phages is Lambda phage.
  Thus i feel now you can understand why sometimes there is no plaque
formation even after phage infection.
 But always remember that  lysogenic phages switch to lytic mode at
sometime during future generation and lyse the cell.
 There is one more category of phage which don`t integrate in host
cell DNA but multiply independently in host cell. These are released
in outside environment without lysis of cell (most probably by
budding) and infected host cell can survive. ex. M13 phage