exactly, some people just can't see spirits. mind you, just because you can't see them doesn't you can't hear them, or sense them, or even smell them, seeing isn't everything. as well, you may not be able to see spirits, but you might be able to see other beings i know people who can see faeries clearly, but need to meditate and focus before they can see dragons. some can't see anything besides one type [like only demons, spirits, faeries, so on]
there are some teas to enhance a person's clairvoyance you could try, also do chakra work, sometimes if the flow of energy is off it can effect things. third thing to remember of course, there's a veil between our world and the spiritual, as the year goes on it grows thicker and thinner, and spirits too go through phases of active and inactive, so you might be picking the wrong time to try and communicate. right now the spirits in my house are relatively dormant, around the middle of april into late may and all of October is when Rori [one of the spirits] is most active. while this little girl [won't talk to me so i don't know her name] is most active in the evenings from july to late september.
don't tend to do spirit work around Yule, [i'm wondering if the little girl will be more active around then] but you could try working with spirits. don't normally go out looking for them, spirits are everywhere really, not many in cemeteries from what i've seen though. try meditating somewhere outside, try asking if there's any spirits who would like to talk. of course, be sure to cast a circle, have some age or something on you, and be sure they don't follow you home. be polite, last thing you want is to upset a spirit, that's never fun.