VI controls are frozen on second run.

I have a VI that I call within another VI. Let's call them VI-a and VI-b.

Now, when I try to run VI-a alone, it works as it is supposed to. No matter how many times I run and close it.

But when it is called within VI-b, it works ok for the first time and all the controls freeze up on the second call (closing and trying to open it again).

What can be the reason?

I am calling VI-a by wiring inputs to connectors in VI-b.