clean cable connections then light on bow connections. then bulbs. any other suggestions.
Make sure you have power to the light and a good ground … 
Just an Old Cracker … With a Gun!
hit it
Anything along the wire path that could impede the current flow to the bulb is suspect. Check all butt splices, switches and connections along the path for corrosion. It is possible to measure 12V across the bulb socket with the bulb out and yet the bulb will still not light when put back in. Each of those corroded connections are resistors which reduce the current in the circuit below the required lamp current.
I had that problem too. Mine would work with the cover off but then I’d screw the cover in tight and it wouldn’t work. I ended up using a lock washer which allowed me to tighten without the pressure on the bulb. Bingo, everything works.
“Sometimes I think it’s a shame, when I start feelin better when I’m feelin no pain…”.
GW 205
F200 Yam
Use a multimeter and test for 12 volts at every connection starting at the battery. Fuse blocks are a prime suspect.
bad ground
Mike
12’ Jon boat twin 300 Verados
Mine was a corroded wire on the positive end AND a bad ground wire.
KeyWest 1720cc 90 Ymha
www.FishCFA.org