I started mining two months ago and within about two weeks of starting I ramped up to 5 GPUs. My first 3 payouts occurred about every 10 days, but the last two have taken 12 days and now the portal shows 14 days until the next payout. My reported hashrate has not changed. I assumed that as difficulty increased hashrate would decrease and it would take longer to payout. But since my hashrate is the same I'm not sure why my payout is different. Is the cause the increase in difficulty? Thanks.
Your hashrate will always be the same. It indicates how many hashes your rig can manage per seconds, it's realted to the computing power of your GPUs. If your setup stays the same over time, this rate is the same. However, network difficulty raises meaning that you have to do more and more hash per seconds to keep the same reward level as it will take more and more time to mine a block. As your hashrate is not increasing it means it takes more and more time for your rig to mine a block. that's why time between payouts raises too.
Note that if you mine in a pool it's the same thing as pools payouts are constant amounts so, say it makes payouts every 1 eth, as it takes more time to obtain 1 eth, the delay between your pool payouts increases. This will continue until delay between payouts becomes infinite.
The only way for now is to buy more GPUs to compensate difficulty.
In a near future, mining system will migrate to Proof of Stake and you won't need GPUs to mine.
On Twitter Jan 6, 2018 Nanopool announced:
ETH share difficulty temporary increased x2
There seems to be some funny business going on at Nanopool. Apparently they doubled the difficulty for no apparent reason. The Nanopool difficulty is changing independent of the difficulty built into Ethereum and other coins that nanopool mines. Users noticed the same thing with Siacoin, for instance.