How to design probe specific hybridize with a specific targeted gene in Southern blot method?

I use DIG-11-dUTP label probe with specific targeted gene to detect in host genome. Host genome have a huge sequence that to date no information in data base. I cut the genomic DNA by restriction enzymes but after southern blot process my specific gene in the host genome does not show the signal band. I use chemilluminence (SDP star) substrate for alkaline phospatase, I do not use radioactive label 32P. Could anyone give me more ideas with this case?