Issue with scala backticks in pattern matching

Question

I have two sealed abstract classes,A and B and their subclasses. I am trying to use them in pattern matching as:

(a,b) match {
   case (`a1`, `b1`) => ....
}

a1,a2...b1,b2.... are subtypes of A,B respectively.

while IDE(Intellij) can detect a1, but it is unable to detect b1. All imports are defined well.

Is it wrong way of using back ticks?