Means the signal level from the antenna is low... if the antenna isn't connected at all, you'd get "No Antenna Detected".
Could be from several sources: poor antenna location, bad connection between the antenna and the dock or between the SL100 and the dock, or bad antenna. Or, you could be parked under a bunch of trees
Wasn't sure whether you were talking about two different installations with two different docks and antennas, or one installation that you've tried two docks in. If the latter, look harder at the antenna installation, try a new antenna, check the connector on the SL100 & make sure it's seating all the way in the dock.
It is possible for the dock to cause that too (two docks, in your case) , but I'd try the easier/cheaper fixes first.
John