I want to talk about a deficiency in a type of fire alarm that is the subject of my research. The deficiency is that when it alarms there definitely is fire in its environment, but sometimes also when it is silent there is fire in its environment. So while its alarm means fire, its silent doesn't mean not-fire. Thus the alarm only cover some cases of fire and its efficiency is kind of one-sided(?). Is there any word for such a deficiency?
The word you are looking for is false negative.
false negative: a test result which wrongly indicates that a particular condition or attribute is absent.
This fire-alarm system is far from production-ready. There are too many cases when it registers a false negative.
Compare it to the far more widely-used phrase false positive, wherein a test result is interpreted by the evaluating system as being grounds for a response, when it actually isn't.
It sounds like the fire alarm is simply not sensitive enough. You could say that the alarm does not have sufficient sensitivity.
capable of being stimulated or excited by external agents (as light, gravity, or contact)