If you're staking your DOT yourself, always keep up with your validators status.

I staked enough dot about 50 days ago to get a daily return and it is mostly a set it and forget it affair. However, it is more like keeping a garden or house plants. I started noticing that that my rewards were in a slump, so I went to check what was going on with the validators on Polkadot.js. Turns out that about 10 out of my 16 validators were now oversubscribed so I was relying on the last 6 to do all the work. Went back through, unsubscribed from them and found 10 new validators to stake with. Costed about .06 DOT to go through that process and is well worth it. I'd suggest if your rewards are flatlining to go and check it at least once a month. Like weeding out a garden every now and then.