Grouping two solids isn't the same as Boolean-adding them. If you've got two objects to be subtracted from, you'll need one object each to achieve your subtraction, ie two copies of the subtracting object (tech term is subtrahend). Grouping them doesn't change that, because if you were trying to subtract from a larger grouping, you'd still need an individual subtracting object for each object within the group. It doesn't work. Add the objects to be subtracted from (minuends) into a single entity and you need one subtrahend only. Or use the option key to retain the subtrahend until you make the last subtraction from the ungrouped, un-added, objects.

Thank you for your reply.

Boolean add instead of grouping soled he problem.

--aryeh