Hi Amitlad,
Interesting, when you said you were injecting volumes up to 200ul, I immediately assumed that this was preparative chromatography due to those volumes but it seems like this is an analytical type column (please correct me if I'm wrong). If that is the case, then something that you definitely want to change is that sample loop and injection volume size. I'm not too familiar with Waters columns, however, Restek (after a quick google search to here:
https://www.restek.com/en/frequently-as ... and-UHPLC/) recommends injections (on that sized column) to be between 8 and 40 ul. Then again, I'm not exactly sure what you mean by "separation unit" and this could be a normal part of your HPLC. However, I would attempt to change your sample loop size first. Another thought I had (because I assumed it was prep LC, which is what I mostly focus on) is that the temperature of your column is not as exact as you would believe. This could be due to things around the atmosphere of the environment as well, like other instruments or AC. The temperature should not be too much of an issue, but I know some columns (with particular acids) actually degrade much faster below their temperature cut-off limits due to those mobile phase additives. It seems like, from your trouble shooting, that the sample loop should be the first thing to try. Now on the other HPLC, was the exact same column used or was it the same type but different physically (aka just another copy)?
I hope you get everything working quickly! I'll be here to help,
Tyler Smith