Follow tutorial to forward SMS/MMS to another phone number
Several issues may cause SMS/MMS to not be forwarded:
Your filter is incorrect because you have limited forwarding to SMS/MMS containing certain content or to be received/sent from/to a certain contact. You may check this in the event page of the app. This page would tell you if the app detected if a SMS/MMS has been sent and why it has (not) been forwarded. You may also use the "Test your filters" option in the app settings page, to simulate a SMS/MMS reception or emission.
If your SMS/MMS cannot be found in the event page, it means the app has not detected your text. It could be because those are not SMS/MMS but RCS, several text app like Google messages, or Samsung messages use RCS message and not SMS/MMS messages which are unsupported by this app. You can disable this RCS feature or use another text app.
Another reason could be that the app was not running when the SMS/MMS has been received or sent You may check the notification tray to verify that the app is monitoring SMS/MMS and when it started to do so. It is supposed to start with phone powerup or when at least one app filter is enabled, so if you see that the starting date is much more recent than those event date or if the notification is just absent, it means that your android system is killing the app. This behaviour is quite common with chineese phones, which use non-standard android. These brands prefer to kill every background app to make their phone looks to have superiour battery life, something which has the disavantage to break background apps (source). You may try tips to solve this issue (here).
