Low mean titer of Mycoplasma synoviae

I would be grateful if somebody could explain me: A flock was vaccinated with Ma 5 and clone 30. Sample collected 3 weeks post vaccination. Using Elisa the response to IB was low although within the expected range. Elisa was conducted for Mg. Ms., ART. In addition to AI, the titer was normal. PCR was conducted only Ms was detected at Ct 29. I would like to ask why the mean titer of Ms was low.