If just one wire coming from a slimline dish than it is a SWM setup. You likely need to add the power inserter and not have a BBC. Also ensure receiver is hooked in the jack that says SWM, only one of the two is correct for SWM.
You may have had it working before without a power inserter, but it shouldn't be done, usually won't work, and isn't reliable. In a SWM with no PI the receiver is powering the LNB, and it does not put out enough power to power a SWM LNB, thus the power inserter. A traditional multiswitch setup is powered by the receiver thus no PI. If you had a SWM setup working with no PI it was luck, and was likely taxing the receiver more than is ideal.