Taking a look at your scores, some of your plays are so bad that they are not actually worth any PP to begin with.
You can see there are 2 different numbers here. The one on the right is how much PP that score is worth. The one to the left of it is how much PP you actually gain, based on how good it is compared to your other scores. The percentage you see in small letters below it tells you the weight of that score.
The bottom 2 scores here are so low that they do not even award PP at all. The score above that is technically worth somewhere under 0.5 pp, which rounds down to 0. Internally, PP can have decimals, but when it's displayed in game or on the website it rounds to the nearest whole number.
When you get a new play in your top plays list, it bumps every play worth less than it down. This means that sometimes it's possible to not gain PP because you gained 1 pp from that play, but lost 1PP from the scores below it being worth less. That is more common at higher ranks where plays might be worth a few hundred PP though.