roberts dab radio shows dls

please tell me why my roberts dab radio will sometimes not stay with the selected station. Sometimes when i switch it on it will display "DLS" with no output. If i change to a differant station in the same family ie radio2 to 5live nothing happens, but if i select classic then radio2 it will tune in.
Two questions,
1. what does the dls mean
2. and how can i prevent it

[Nick_UK]

That's something you'd have to refer to the manual for.

Digital Loss of Service ? Sounds like you need a better aerial.

cheers thought it would be something like that.