group in assignment does not work

I have problem in setting assignment to be available to one group only, I am learning moodle

moodle Moodle 2.4.3+

I have course with teacher1 and 2 sections( A and B) 

I want teacher 1 to add assignment for only section A

I did the following as an admin :

1)course setting >users>groups

I creat Group A and added  students , and creat Group B and added other

I create grouping A, and  added to it group A, and grouping B and added to it group B

2)course setting >groups>

group mode :seperate group

force group mode: No (I tried yes)

default grouping: none

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then I added assignment, in assignment setting

3)Common module settings:

Group mode :seperate group.

grouping: grouping A

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the result , the assignment is available for every one even in group B

the option "available only for group member" is not in the setting

how to set this right

thanks in advance i really need help, it took 2 days until now