It is not abnormal. I use to see spirits / ghosts all over the place but now I only see them once in a great while. Mostly just out of corner of my eye. Also visions of future events come randomely. Could be that you are clouded. Meaning you have negative energy or events blocking your gifts. Could be you are trying to hard.
Maybe it is because they do not want to be noticed.
If you are just now developing senses to detects spirits (whether it be hearing them, seeing, feeling their energies), then they will know that you cannot detect them. Now once you're able to tap into your developing senses, they'll detect that as well, and then *poof*, they allow themselves not to be seen.