Ok, so here's a bit about how PP works.
In order to figure out how much PP you get for something (which osu just ranks players from most PP to least), it doesn't just add up every single play you make. That would make it no better than how the score system used to work before PP was introduced.
Instead, the PP calculator for players looks at your top 100 best plays. It ranks them from highest PP to lowest. Then, it reduces the amount of PP you get from each score as it goes down the list.
Your highest PP score is worth 100% of whatever it says the score is worth, but your next score is only worth 95% of that PP value. The next score after that is worth only 95% of 95%, or 90.25%, and it keeps going down the list.
If you had 100 scores all worth 100 PP, you'd only get 1888.75 pp instead of the expected 10,000.
This means that if you haven't gotten better in a little while, and most of your top say... 20 scores or so, are pretty close together, you're not going to get much PP at all unless you get a really good score on something hard (for you). Fluke amazing scores on stuff will give you PP, but an average play on something probably won't help any more.
Basically, this means that the better you get, the harder it gets to actually get PP from a score (unless you're so good that you can get an amazing score every time a REALLY high SR map gets ranked)
You CANNOT farm plays at the same difficulty level of maps and expect to get a higher rank, you need to actually get BETTER scores than your top scores in order to gain any reasonable amount of PP.