I play PP regularly up to level 180. The "poor rewards screenshots" several people have posted here are actually regular level rewards. When you get to higher level positions the base reward is much bigger... So these screenshots are not payouts and not a sign that PPs are not working.
The rewards ALWAYS drop on low level positions if you're in reward mode. It's a 100% effective test to verify whether you have paid back your debt to the PP bank or not.
Attack a level 1-20 position and calculate if the reward is bigger than the loss (in terms of resources). For example send 50 Thureophoros against a level 12 position. You lose 1 Thureophoros and win 1 Cretian Archer means you're still in payback mode and you CANNOT drop a big reward. Be careful Plarium sometimes gives you rewards that appear "big" but are not - like 30 swordsmen or 50 psilos. A real reward is typically 10-20 times bigger than your losses.
If you lose 1 Thureophoros and win 5 Mounted Peltasts you are in reward mode and a big reward MAY drop on any position. Chances of a reward drop in this mode is nearly 100% up to level 50 positions and then chances drop off. I would guess that odds are about 20% at level 150 and up.
If you have verified that you are in Reward mode it is almost certain that a reward will drop within 10-20 positions cleared. But it's still chance. I one-bar every position from 150 to 180 so I have enough positions ready to clear that I will be able to get the reward. Once I have verified I am in Reward mode, I start from the lowest position I am willing to get a reward on (150 in my case) and work up until the reward drops. I do Off or Def positions depending on what type of units I want to win. I've done it 100s of times and it has NEVER failed.
I have done it multiple times in a row (win a big reward, kill the entire reward + 10% and get another reward immediately) and it still works. I dont think there is any waiting period but of course when you win 10,000 Agemas you need to kill 11,000 Agemas + all the units you get as small rewards in between before you return to reward mode. If you do the check on a low level position as described above and don't show reward mode there is NO POINT in trying to get a reward. Reward chance is ZERO until the balance is paid.
I have had extreme bad luck at times, when it takes 10 positions to get the reward. I also frequently drop it on the first or second try. If all the positions at the high end of the range are one-bar-ed it's no big deal to hit 10 to get the reward. Make sure you have enough positions one-bar-ed before you start.