Periodically DEPC treat the plasticware again, especially when there is a problem with the in situs. Gloves are worn at all times during prehybridization steps. Proteinase K digestion may vary with probes and tissue. It may be wise to do a series of time points to determine the optimal length of digestion. 30 seconds - 5 minutes would be a good start.
Signal detection is dependant on the concentration of probe in the hybridization solution.
If you get negative results first try repeating the immunocytochemistry, be sure that all your buffers are at the correct pH. The alkaline phosphatase reaction is especially sensitive to the pH of the buffer.