Refx Nexus Volume drops

I used Nexus for a long time without any trouble.
But recently the volume of Nexus drops to a very low level automatically.
It plays at normal volume for a few seconds but then it drops suddenly and the only way to get it louder is by moving the slider of mixer all the way up and then it is still very soft.
It doesnt happen with any other vsti so it has something to do with NExus settings i just cant figure out what is the exact cause of the volume drop.
I hope someone can help me with this problem because nexus is my main vsti I use it allot.