Hi

I would like to update the opportunity stage to "transacting" when a roll-up summary field on the account (Total Transaction) become =>2 which is getting information from bill object. I tied many ways but unable to change stage. 

I tried below ways none of them are working.
1) Roll up summary field won't trigger workflow and process builder
2) Created a checkbox on opp which turns "true" when total transaction meet criteria, But still won't change stage through Process Builder/workflow. 

Please help!!!