Hi,
It seems you have not associated entity B record to entity A.
When we create a 1: N relationship between entities in CRM, it creates a field of type “lookup” in background on Child entity.
This lookup field points to the Parent entity and it will be created with the name which we specify while creating relationship as shown below:
For egg. If we create 1: N relationship between Account & Contact where Account is the parent (1) and Contact is the child (N). So now after creating relationship, a lookup field of Account (parent) will be created on Contact (child) and this lookup will point to Account (parent).
So now in order to show the Child (Contact) record inside the Sub grid on Parent (Account) form, you will need to associate Child (Contact) to Parent (Account).
That is nothing but lookup field on Contact (child) should have Account (parent) record selected in it. Then only that associated Contact record will be shown in the Sub grid which is on the Account form.
Note:
While configuring sub grid of Child (Contact) on Parent (Account), make sure to set right properties i.e. Entity selected should be the related Child entity i.e., Contact in our case and “Records” property should be set to “Only Related Record” as shown below:
For more details you can find the below link:
Hope that helps!
Thanks