This may be due to the presence of an immiscible solvent remaining in the column.
Even trace amounts of these solvents can cause issues. The immiscibility manifests as an unstable baseline, such as the example shown below. In order to fix this issue, try flushing the column overnight with a mutually miscible solvent such as isopropanol to remove any immiscible non-polar solvents that may be in it.
This phenomenon can happen to Cogent TYPE-C™ columns or others columns that may have been introduced to immiscible solvents. In some cases, you might observe this with a new column if residual packing solvents may still be present.