mieke,
I've read your post from yesterday. The question RCJ was referring to was what happens to the mitral regurgitation in the setting of exercise. If it doesn't increase or other abnormalities develop (ie cavity obliteration) then as stated it isnt necessarily common, but can happen.
The pressure not rising is not of concern unless conditions such as above are developing. The reasons why some people have a blunted response in the setting of what appears to be normal cardiovascular function is unclear, but probably not of great significance.
Glenn