did you try turning the bulb 180' in the socket? some times that works.
it may also need to be a "flasher" type led... not sure on that though

Some LEDs don't light in the early Williams pops that are under the CPU control. I think they strobe at 18v or something if I remember correctly (check with a meter in the RMS mode and correct me). The strobbing makes a kind of 6v.

Try a more complex LED with a regulator chip in it, those should work fine.

I looked through some pictures on my laptop and the Cointaker frosted dome LEDs work in Sys3-7 CPU controlled pop lamps, the "Ablaze 1-LED #44/#47 Bayonet Base Lamp With Frosted Lens" do not.

interesting

then you will probably will like how this looks (all LED and extra LED Starpost LED)

and with turned on room lights:

LEDs in the pops is one of the few places i think they look good at.

I have some in my Time Warp. I used the 5 led from cointaker (2-3 years ago). Work fine, but i had to do work on the sockets in the pops. I had to adjust the bottom thing closer to the tip of the bulb as the spacing was slightly different than a standard 44/47 bulb.

Those green gems by the snake pit look awesome. Gorgar is a great game to bling out.

In my popbumpers are single Led from SuperBrightLed.
No problem fitting in original sockets Ba9s