It could be a number of things. Two that spring to mind are checking your aftertouch settings for the instrument you are using. Just to make sure you're not filtering aftertouch.
As for the Impact instrument, the pads you are playing could have "choke" enabled. This means if you play a pad then follow it by another pad, the second sound will choke or silence the sound. Very useful for multi-timbral drum samples set to velocity. Choke works in groups. So you can have, for example, a hi-hat open sample played closed hi-hat sound, and have the open sound "choked" by the closed sound, stopping the sound being unnatural.
You don't mention which pack you are using to compare the issue, so I can only leave you these two suggestions.