Up at the top of the lamphouse, there is a knob that looks like a second focusing knob. Assuming that you're enlarging 35mm negatives, turn that knob until the lamphouse is all the way at the top of its travel.
Above the condenser of some Beseler enlargers, there is a trap door that's used for inserting filters. Open that trap door, if there is one, and check to be sure that nothing is in the slot which might block the light.
Open the enlarger gate and look into the slot where the negative carrier goes. Can you see the enlarger lamp through the slot? Is there anything in there, other than the negative carrier or film which would block the light?
On some Beseler enlargers, there is a small, silver handle to the left of the lens that moves a slide-out filter holder. Slide that handle all the way to the left (from the operator's usual perspective) and ensure that there is nothing in the filter slot.
Finally, from the perspective of looking up at the lens from underneath, turn it all the way clockwise to its widest (lowest numbered) stop.
What we have done is to start at the top of the system, where the light bulb is and work our way downward until we get to the lens so as to ensure that light is getting through each stage of the system, as it should, until it reaches the baseboard where the photographic paper should be placed.
