What could be causing the low water level in my shower trap?
If your shower (or any drain) is draining to your sump pit, you are overworking your sump pump and shortening it's life span. If you have a radon mitigation system that uses the sump pit, you are also creating a situation where radon can potentially enter your house.  The only water that should ever be in the sump pit, is ground water.
Aside from that, here are a few things that may cause your issue.
Evaporation
If the shower is not used very often, the trap can naturally dry out due to evaporation.
Solution
Either use the shower more, or routinely pour water down the drain.
Blockage in dry vent
If the vent becomes obstructed, water flowing down the drain can produce a syphoning action and suck the trap dry.
Solution
Clear the blockage in the vent, by snaking out the vent pipe.
Blockage in wet vent
In situations where a vent is not directly connected to a drain line, a wet vent will be used.  A wet vent, is a pipe that serves as both a drain line and a vent.

If this pipe becomes restricted, syphoning action similar to a blocked dry vent can suck the trap dry.

Air cannot get past the obstruction while water is flowing through the pipe, which creates a pressure difference that must be equalized. There are two things that can happen in this situation. Air can be forced past the obstruction (possibly causing gurgling), or air can be forced through the trap (causing gurgling that you're more likely to hear).
Solution
Snake the drain and clear the obstruction.
If you really are draining to a sump pit used in a radon mitigation system
If both drains and a radon mitigation system are connected to the sum pit, the radon system could be sucking the water from the trap.  A radon mitigation system works by sucking radon out from under the house, and venting it outside. If the pressure in the radon vent is lower than the pressure in the house (which is sort of how they work, so it probably is), the water in the trap could be forced out and down the drain by the higher pressure air in the house.
Solution
Don't drain to the sump pit, especially if you have a radon mitigation system.