My rating is not increasing

I have solved 6 problems out of 8 in division 4.My rank was 1200 out of 9500 participants.But my rating get down from 1344 to 1310.I wants to know why? Do I have to solve all problems?

If you check problem difficulties you will see that the hardest problem you solved is roughly 1300 so it makes sense that your rating stays roughly 1300.