Why after the iteration of 20000 times, the loss oscillates around 1.4, no longer reduces

[HappyKerry]

I used 512 ssd training, iterating about 20,000 times, the loss dropped to 1.4, the map is about 0.9, but then the loss starts to oscillate between 1-2.5, and the map no longer improves.why？

[HappyKerry]

@chuanqi305