Related entity add existing button is not shown on grid

I am facing an issue i have two related entity record with entity c.
When i click on sub grid of hospitalization i can see Add Existing Hospitalization button but when i click on lab test grid i can only see Add New lab test not add existing button. The relationship between the c entity and hospitalization entity and lab test entity is same. (N:1)
I want to show that button please help any help will be appriciated.
The button is not hided from ribbon customization. (there is no change in ribbon)

Thanks