It's just speculation on my part, but this is likely just due to your fluctuating revolving utilization. And assuming that you are an AU on the $38K card that you share, it would be included in your revolving utilization as well.
Also it is not a given that your FICO score will increase every month. I have seen situations where my FICO score remains flat for months at a time, despite my baddies aging, and continued on time payments on my part.
You re not reporting any baddies, ie charge offs or collections. Are your unpaid balances fluctuating a lot on your revolving credit, that is your credit cards? Meaning are you pif and charging or increasing a few hundred or thousand either way? This will throw Fico off. I can speak from experience. Fico only sees unpaid balances, not that you pif and then recharge so to speak. My score has taken a hit from TU and EX because they see my monthly payment rising. Fico assumes that you do NOT pif each month. It has no way of knowing if you do as it recieves information only from cras after the fact, not real time. If you are pif each month and then recharging/using credit, I would not be concerned unless you are apping for a home mortage. As always YMMV.
Are you pre paying all of credit credits in full prior to the end of the billing period ( statment date)? If so allow one or two cards to report a balance.