Firstly which type of problem you are noticing??

Check the dp. of the Heat exchanger.

write the bottom col. & top col. Pr. diff. , if possible plz. mfg. Co. of the Distilation column with Heat Exchanger ???

Without knowing any details of your process, it will be difficult to suggest anything. Is it a brazed aluminum heat exchanger? Usual suspects in poor heat exchanger performance in a cold box (which I think you are suggesting) would be frozen components plugging the tiny passages in the heat exchanger. These are usually water or CO2 in my experience, but I don't know what your inlet conditions are like. As another poster is suggesting, check the DP across the exchanger as this is a dead giveaway.

Are you not meeting spec on the outlet gas? Are you noticing any car-sized ice balls on the outside of the cold box?

Could you warm it up with dry N2 and run a moisture and/or CO2 analyzer on the nitrogen discharging the unit?