I have a 150L dry cabinet with a electric hygrometer. I have set the humidity to 45% rh but the digital meter is showing 36% rh. The dry cabinet is in air conditioned room( which operates occasionally). I cannot understand why rh value is falling below 45%, previously during the winter season it was working perfectly. The rh values stayed in the range of 44-45%. I am at my wits end as to why it is malfunctioning now. Is it a technical fault or an ambient temperature related issue?
I'd guess that your dry cabinet probably only dehumidifies. It is kind of like if you have your AC turned on and set to 70 Fahrenheit. If it is 32 degrees outside, you're AC doesn't do anything because it isn't needed, but your temperature is going to drop much lower than 70.
Similarly, your room has a lower relative humidity than you are trying to maintain, so there is nothing for the dry cabinet to do. It is displaying that the relative humidity is already lower. You would need a humidifier to bring the humidity up in the box if you needed to actually regulate it exactly.
Alternately, it could be that the meter is actually broken, you could try putting in a wet sponge as BobT suggested below and see if the value goes up or not. If it does, then the room humidity is "probably" just lower than you have it set and it is working ok, if the value doesn't go up, then most likely the meter itself is broken and may need to be serviced or replaced.