After years of working with some very dedicated people, I have discovered a reason they cannot perceive spirits except in unusual circumstances.
They are continually being overwhelmed by spiritual chatter and cannot detect individual messages except under unusual circumstances.
Upon inspection, it appears that they have no concept of what it means to experience spiritual silence. They experience spiritual chatter as other people experience tinnitus, a continuous inflow of thoughts which they cannot seem to control. (Tinnitus is the hearing of sound when no external sound is present. )
One should learn to meditate or to train one’s attention until one can experience spiritual silence. Just sitting quietly and being there without thinking will do this after many hours of practice. Once you have experienced spiritual silence, then you will be able to recognize thoughts that are yours and those that are not yours.
The people I have observed who seem to be spiritually deaf are simply overwhelmed and cannot pick out communication directed at them from the incessant spiritual noise they are buried in.
If you have difficulty detecting spirits, make sure you are not simply buried in spiritual noise. Meditate until you can detect spiritual silence and then try contacting spirits.