got 3 homeplugs on a home network -

when (for testing purposes) I plug a fourth in to a socket near where
an existing homeplug is plugged in, one of the other distant homeplugs
stops working properly (IP cam - no picture). But if I plug the fourth
one into a socket further round the *same 240v circuit* as before- all
is well.....???? any clues/thoughts please?

Cheers
Jim K