Have you tried to increase the fan RPM? If you do not get the desired result even on the maximum RPM, can you please remove the device and check the flow before the installation. Also, check and see if the air coming from the vent is warm/ cold enough.
In some houses, the vent is simply disconnected from the main duct branches. There are several causes for this including a bad renovation process or loose duct joint which leads to disconnections over time by natural structure movements. Whatever the cause is in these types of situations, the booster fan cannot be effective. It is important to mention that our device cannot generate energy (heat or cool) on its own but increases the energy flow which already exists in the vent. So if before the installation, close to nothing flow coming from the vent the device cannot be effective. Please consult with an HVAC/ renovation contractor.