It's been WAY too long since I worked with an EL (a favorite camera)
but if I recall correctly:

In cameras like this shutter speed is determined by charging a
capacitor, and when the capacitor is full, the shutter closes.

The rate of charge is set in different ways, depending on the mode. In
auto, the current is set by the meter - more light, more juice, faster
shutter closure.

In manual, the current is set by a variable resistor under the shutter
speed dial - a "thin film resistor" I believe. This is a disk with a
variable coating of conductive material, and when you turn the dial a
slider contacts varying positions on the coating.

Over time, gunk collects between the contact and the disk; and/or the
conductive coating itself deteriorates. The upshot is, sometimes you
get the right speed - sometimes you get a slower speed - and sometimes
the circuit is open and the shutter never closes at all.

This isn't unique to the EL, or even to Nikon - I've seen it on Olympus
and Pentax cameras, too. This is the problem that has disabled many a
Nikon FTn meter prism.

Sometimes, you can ease the situation by cleaning the resistor disk,
but this is usually temporary. The real solution is to replace the
resistor. But there aren't any available - at best, you'll get one
salvaged from an older camera, and may soon be back in the same boat.