Why ph of DM water at the outlet of Mixed Bed starts dropping after certain time?

The ph at the cation bed outlet is 3 to 4.5,The ph at the anion outlet is 8 to 9.0.The ph at the mixed bed outlet is 6 to 8.0.But after a while ph starts dropping it drops upto 4.What would have been the probable reasons.How does the deger operates.Whether it part of DM water plant