This is because thermal paste loses its properties after 6-12 months, depending on temperatures. If temperatures are very high, you'll need to change the thermal paste even more frequently.
Monitor your device after 6 months, and you'll see that temperatures will rise again. Some devices have low-quality thermal paste from the factory, and miners immediately replace it with high-quality one.
Good quality thermal paste such as Noctua NH series or Arctic MX series are good for well over 12 months. The factory fitted cheap stuff typically a chewing gum pink colour does dry out pretty quickly so its worth the extra price and effort to get something that's reliable.
I've used Arctic MX-4 on most of my rigs and they've been running for about 2 years without any noticeable change in the thermal performance.
I recently changed the paste on one of my Bitaxe Gamma boards to Noctua NH 2, that's now running a good 8-10C cooler than it used to.
I use Noctua NH2 in one of my PC's and that's been running without any issues for over 5 years.
So upgrading thermal paste is a good idea, I've also noticed that due to the lower temps the Bitaxe is now running 0.25W less power draw as well; so better efficiency all around.
Still doesn't help finding a block though!
