Problem passing variable to function in ISR

So, I have a problem sending a value to a function at different parts of my code. Here is my code:

void LongRightShift(unsigned char NumBitsShift)
{
// My Code Here
     NumBitsShift = NumBitsShift; // So you can place a breakpoint here and read NumBitsShift
}

Now, when I call

LongRightShift(2);

in the main part of my program, NumBitsShift = 2. When I call it in the Decimator2_ISR, NumBitsShift = 1;

Does this have to do with the way I'm passing it or is there a conflict with running other functions in an ISR?

Any help you can give me would be appreciated. Thanks!