You have not posted enough information for a conclusion to be drwn as to why your having an issue.
How do you have your setup wired?
For example do you have a cable box? If so are you connecting a splitter to the incomming cable line (from the wall) then connecting one of the splitters outputs to the cable box and the oother to the slingbox pro hd's coaxial input? If that is the case, you may have insufficient signal strength comming in on the cable line which is causing th issue in which case you would need either a service call from the cable company or amplify the line before the splitter.
As I said, plase describe your specific setup senario for more elaborate help on this matter.