If the retention times are unstable across a plate or shifting over time:

• Make sure the mobile phases are purged and there are no bubbles in the system.
• Make sure no air is drawn or injected by the autosampler.
• Make sure the entire LC-MS system is well conditioned and the column is sufficiently equilibrated.

Otherwise, check the following parts with your instrument engineer:

• Degasser
• Mixing chamber (needs to be replaced?)
• Intake/outlet valve at pump head
• Seals at pump head
• Replace the analytical column (it may have reached the end of its lifespan)