It sounds like a carryover.
Try to run a blank injection after every sample. If this doesn't help try to wash your column with a different mobile phase which clean it of.
You have to see if 40:60 ACN/H2O helps. After it you do a blank run to equilibrate the column to its starter conditions.
If you have the problem that the concentration increase over a few days, then try to put a wash program after every sequence.
For example 10/90 ACN/Water for like 1 hr. Just write a method which runs after each run and uncheck "save Data or put Signals off"
After this 95/5 ACN Water. So u retain the storage conditions.