This sounds rather like the problem I reported 31 July 2010 [J731-001];
if so, it's to do with font selection.
> On Mac OS X Snow Leopard, GNAT GPL 2010's GPS 4.4.1 doesn't find the
> Lucida fonts; the result is odd-looking, not least because blank lines
> are squashed: see lines 22/23, 25/26, 27/28 etc below.
>
> Monaco is a much better choice (at points 10/9 rather than the default
> 12/10).
On this machine (Lion) I have the fonts set to Inconsolata, but I think
that was a download from <somewhere>. Monaco certainly works for me.
In Edit/Preferences, there are 2 font settings unger General and one
under Editor/Fonts & Colors - the last is the one that controls code
appearance.
HTH