If you are experiencing different results (taste of bitterness, then no taste at all) with the PTC paper, there might be a few explanations.
Sensitivity to PTC is based on a genetic disposition, so it seems likely that a positive one time should give a positive the next time. There does appear to be evidence that results with the same individual can vary as much as 8 times. This may be what you are experiencing.
It does make sense to limit intake of food or drink prior to the test, since it may be possible to offset or mask the effect. Have a supply of water available to rinse out your mouth (spit) to eliminate the bitter taste. There is also mention in some literature that smoking can diminish a person’s ability to detect PTC.
PTC paper is very stable, so it’s unlikely the paper is too old to work. A couple of other unlikely possibilities might be that the paper didn’t get proper soaking treatment during manufacturing or the paper was Control paper and not PTC. These situations are both very unlikely, and could be tested by trying a strip from another vial or batch.