Double space instead of single space

[icebna]

LyX-1.4.3 in Mandriva linux

Why some paragraphs have a space double of which it has been fixed ?
For example :

Bla, bla, bla, bla
Bla, bla, bla, bla

Bla, bla, bla, bla ( double space instead of single space )

Some idea

Thanks

Miguel