You'll have to remember some basic rulles... here are the reasons:

1. They had shutedDown their comps.
2. They are not sharing any files.
3. They may be using other p2p programs and therefor# are a little hard to connect to...

For the 2nd ones you can limit their acces to your computer:
tool->options->advanced->preferencing
change the files you must share... to 10...100 and drag the slider to Rarely