I would suspect from what you are saying that you get good views when the hubs are new and showing on the HubPages feed but start to fall after that.
To give your hubs the best chance of getting consistent traffic, advertise your hubs on Social Media such as Google+, Pinterest, Facebook etc. Writing new hubs that are related to your older hubs can also drive traffic to them if you link them together.
Sadly, there is no easy answer and your hubs will end up finding their own level based on the amount of people searching for the topic discussed in your hub. But you can help them to make the most of the traffic available.