Replace with a 47 socket. If it is not that try another light board.

I'd be thinking SCR if the new socket doesn't work.

I have an EBD (1984) that uses the 555 sockets. Those sockets are 1000000000x better than the garbage bayonet sockets Bally used for years prior. There are two very common causes for bulbs not illuminating. 1.) The SCRs. The lamp and auxiliary lamp boards are unchanged from the early Bally SS games. The 2N5060 and to a lesser extent MCR106 transistors on the lamp board are highly prone to failure. Schematics clearly tell you which "Q" number controls that lamp and they are very easy to test with a multimeter and very inexpensive to replace. 2.) The pins on the lamp/aux lamp driver board are prone to cold solder joints. My "1 ball" lamp would not illuminate despite the socket testing good and the chip/SCR testing good. I was completely stumped only to notice the lamp turned on in all-lamp test when I jiggled the board connector. The solder joint was so bad that I was able to pull the pin out with my fingers!

It's almost never the socket on these games. The 555 sockets are actually really good.

Which lights? Originating from the main lamp board, or the auxiliary lamp board?

I've been troubleshooting lights in my Eight ball Deluxe. I ordered and Altek lamp driver to act as a "control" to help me understand what parts of the machine are malfunctioning. For some of my lamps that just would not light after checking for good bulbs and good 555 fixtures, I've traced the problem to specific transistors on the lamp board. I watched this video ton how to check transistors:

Sure enough the lamps that won't come on correspond to the specific transistors on the old lamp driver board for those lights. I've got a $15 ebay repair kit inbound so hopefully I can make the old lamp board work correctly and save my Altek board for troubleshooting on other machines.

Another alternative is Ed at Great Plains Electronics. https://www.greatplainselectronics.com/categories.asp?cat=17

Same with my original Centaur and an Elektra I had for awhile. Two or three problem sockets on each when I got them. After cleaning and squeezing the contacts closer together, no problems. No flicker, no dim GI.