You didn't use polybrene or other polycationic substance to enhance infection. You don't know how to calculate the titer properly or your X-gal or other reporter system for titer calculation is not working properly. Ensure that the cells are in optimal growth media at time of infection. Target cells might not have been healthy. Target cells were not murine cells and you used an ecotropic envelope producer line.
Target cells are dead.
Target cells might have been killed by polybrene. This is especially a problem with some B cells and certain T cell lines. Lower the concentration of polybrene. Target cells are murine T cell clones , which are sometimes particularly difficult to infect with murine retroviruses. Target cells are EBV-derived human B cells , which also are sometimes difficult to infect efficiently.