Is this ONLY when using your phone. If so then it has something to do with your phone setup. If you are not receiving email notifications when using your phone or computer then it is your settings for the subscriptions.
To change your subscription settings go to Quick Links, scroll down to Subscribed Threads then click. Once there either click the Notification box at the top right of the subscribed threads or click the box on each thread individually. Next, move to the bottom of the subscribed threads where it says Selected Threads and click the down arrow just to the right of Move to folder. Select how you want to be notified then click GO.
If you want to change the notification method for all future post then change the setting under User CP and Edit Options. You will find a section called Messaging & Notications then Default Thread Subscription Mode