There are 3 possible causes:
- it could be a contamination of column or guard column and you have to replace the guard column.
- if the problem still exists your column could be plugged and you have to follow the conditioning or recovery procedure mentioned in your column's manual.
- if the problem still exists the inlet frit of your column could be plugged and you have to replace it.
There is still a chance that the problem persists but then it can only be solved by replacing the column.