The same thing did happen to me. I would suggest to keep practicing. And also try to increase accuracy. Do upsolve the questions which you were not able to solve in contests. It takes a little time but after a few weeks or maybe a month, you would surely see the reflection of your hard work in rating.

Edit: I just saw that you only gave 8 contests until now and the last one you gave was in March. How do you expect to increase your rating if you're not giving contests?

Maybe try practicing in virtual contests? Or it could just be stress / anxiety from contest making you lose performance.

It also happens with me, I can't solve D in the contest most of time but after the contest I can easily solve it. The only thing which can be done in this case is that, we have to do it, we have to push overselves at least once and I think that our habit will change (mine is only solving till C in the contest) and new habit will come (which will be solving till D). Although This should not be like this that there should not be habits. It is no custom that a newcomer must solve at most 1 problem or specialist/expert should solve till C/D. We should go for the change or we will never be able to improve ourselves. Don't lose motivation during the contest and keep working even if only a minute remains.