Comparator doesn't output redstone-signal

Somehow the comparators (randomly) don't update if the redstone-signal changes.

What I expected:
1. Comparator compares input A and B.
2. When A>B, or A=B the output turns on

What actually happened:
The comparator gets powered, but it won't output a signal.

How to reproduce:
1. Place a comparator (works in every direction)
2. Create Input B with a bigger Redstonelevel than Imput A
3. Create Input A
4. Remove Input B
5. Comparator doesn't update.

Important:
Create Input B before A.