Could be one of two things, first your disease progressing/getting worse which means the remi proteins in your bloodstream are all used up before you hit the 8 week mark, necessitating a higher dose.
Seccond, some do develop antibodies to remicade over time (remi is part mouse protein and your immune system can target remi and develop antibodies making it partially or fully inert/useless). Often remicade is taken concurrently with an immunosuppressive medication such as 6mp, imuran, or aza to help prevent antibody buildup in the bloodstream; I don't see one listed in your signature. Taking a higher dosage of remi can counter this in some patients.
Prometheus labs offers an anserifx blood test that checks remicade aka infliximab concentration in your blood a few days prior to your next infusion.
www.anserifx.com/ That test also checks for antibody build up in your blood. It helps discern the reason you need to bump up you remi infusion frequency. However not all insurance covers it, and it can be pricey. Labcorp does offer a similar test, you might have better luck getting that covered by your insurance.
If it is antibodies, you may have to eventually switch meds to another biologic (e.g., simponi, humira, entvyio).