The Dash 8 has a little peculiarity in this case. Over the Copilots seat, there is a flap door that hides the manual main LG release handle. Opening it will disconnect the hydraulic pressure from the main LG. So if it has been opened for any reason, for example during maintenance, and not properly closed again (a few degrees are rumoured to suffice here), the main LG will not retract on selection. The copilot is supposed to check proper closure of the door during his preflight, but every once in a few years, this is overlooked, as things happen.
A quick and easy remedy will be a quick slap (applied to the door!); more often than not this will result in the landing gear resuming its normal operation.