Sometimes if only some peaks tail, it is a new, small peak that is coeluting with the tail of the larger peak. A diode array detector can help determine if the compounds are the same.
If all peaks are tailing, It may be a problem with extra column effects from tubing or fittings. Check/change tubing and fittings.
It may also be a bad column or the wrong phase for the separation. Try a different column/phase.
A build up of contamination on the column inlet can also produce the tailing due to secondary interactions with the contaminant. Clean the column with a series of solvents and gradients, or change.
As we saw earlier, heavy metals can cause this as well. Try a more pure phase or acid wash the column.
