Check to see that the correct signal is assigned. You can assign not only detector outputs, but detector, injector and oven temperatures, as well as pressures to a signal. When you verify the signal assignment, check the signal level. You want some number greater than 0-1. This check is important especially for a TCD or FID, to ensure the wire is intact or the flame is lit. Check the column head pressure. If it is lower than expected, check for leaks.