Kiln inlet draft going on higher side, and drop in production by 10%

Dear Sir,

We have 500 tpd kiln (3m x 42.5m length) with 5 stage suspension preheater system, we are getting only 450 to 460 tons  (we have comfortably achieved 500 t + in the past), now all the parameters maintained remains same which we have maintained earlier, but when we increased raw meal feed rate, kiln inlet draft is going high and feed rush towards burning zone, which will force to reduce rpm, it has been observed that the inlet chamber is clean and there is no blockage(narrow ring) anywhere observed, burning zone coating is uniform, what could be the reason that the raise in kiln inlet draft and corresponding drop in inlet temp, but the cyclone temp are normal, 

Due to the above reason, our clinker production drop down by 40 to 50 tons in a day, I request anybody to help in this regards.

Ramesh Aital