What Is Causing Engine Oil To Evaporate Or Disappear In A Month After An Oil Change?

As nutcase stated above you may not be putting enough oil in to begin with.  Most of the time when people change their oil they will replace the oil filter.  Which is only really recommended every other time the fluid is exchanged.  When doing so most don't put into account the amount of oil that is retained by the filter.  When you change your oil and filter you need to fill to the max line on the dipstick.  Then start your car and allow the pump to fill all the lines and filter with the fresh oil.  Allow up to 5 minutes of idle time to ensure proper dispersion of oil throughout your motor  and its plumbing.  After shut your vehicle back off and recheck your oil level,  it should be about quarter of an inch to an eighth of an inch below the top line.  Top off the oil and you are done.  If this doesnt solve the issue you may ave to take it to a gas analyzer to truly determine if it is burning off oil or not.

There is a few things to check. You have checked for an oil leak and not found one I assume but check your cooling system the oil might be going there. Or the simplest answer is are you putting enough oil in? sounds dumb but the oil does need to work its way though the motor. When I do an oil change I always fill the oil up let it settle warm the engine then recheck the oil level.
If that proves unsuccessful I assume the car is burning the oil, which means your engine is running to hot so best bet is to take your car to the garage and get them to check the car over.