For the S.W. side of a M/E L.O. cooler it is noticed that rise in S.W. temperature through the cooler has decreased from 12 deg. C to 7 deg. C over a period of time, while the drop in temperature of L.O. across the cooler has also decreased. All other coolers have been reported to be working normal & M/E is also running at a constant load in calm weather. What is the inference drawn by decrease in the temperature difference?