I've built a website for a friend, which has been tested on my MacBook Air, mobile devices, a 21" monitor and a 42" screen as well. Throughout each browser/device, it all seems to have looked alright.

However, I've since been told that there are a few issues with the site on an iMac, which is roughly the same size as my monitor. No matter which browser or monitor I use, I cannot replicate the problem - but I can see from the screenshots sent to me that certain images, slider buttons etc are not where they should be etc.

Is there any reason this might be happening specifically on an iMac? And how do you go about fixing a problem you cannot replicate?