This is to be expected as the review campaign asked a much larger audience of patients initially and has now excluded those patients for X days based on the settings. If you have a lot of long term patients there are only so many patients to ask each day who have not been asked already. The X day exclusion setting can be lowered to start asking more patients right away.
Note: if you have previously used another software for online reviews it is normal to see a decline in new reviews over time. Initially the other software inundated all of your patients with review asks and captured reviews from patients who were willing to leave one. This means that the initial audience that was asked for reviews was much larger than the existing group. Obviously the more new patients you see who stay for care, the more opportunities you have for new reviews!