Add Existing Does Nothing

I have created a n:n relationship between account and product, but I am unable to add an existing product to an account. I am following these steps.

1. On account form, go to Product Associated View click Add Existing Product,  Look Up Records form is displayed.
2. Select one or more records, they appear in Selected records list.
3. Click Add.

None of the records are associated with the account, no JavaScript error and no errors appearing in the trace.

Any ideas?