Check the flipper board in the cabinet.
Most likely it has bad header pins.

Which flipperboard is now in the game?
It should be a 520-5076-00 board.

Do the led's blink on the SSFB when you press the flipper buttons?

The switches from the SSFB are done electronic.
Here is a part of the schematics which show the 3 transistors for each flipper.
The red line is the "Switch Drive" and should be the Green-Gray wire.
The pink line is the "Switch Return 8" and activates sw. 64 (White-Gray wire).
The green line is the "Switch Return 7" and activates sw. 63 (White-Violet wire).
The blue line is for the upper flipper and isn't used as you can see on the left side of the image.

If neither of your flipper switches register, check the green-gray wire for good connection.

For the record, 520-5076-00 is a modified 520-5033-03 board.
Same functions, but with an added safety for failing EOS switches.

Peter
flipperboard02 (resized).jpg

Nope, wroung connector.
These are the coils.

You need to check CN8 (more to the right).
Check if the wire is firmly seated.
Like Bobukcat said, check the whole column for good operation.

Data_East_CPU_520-5003-02_layout (resized).jpg