Country exclusion from coefficient expected, but number of groups decreases!

Hi,

I have a sample of 24 countries, that I want to use in my fixed-effects regression.
In my results table, that should be the 'Number of groups' shown.
My panel is unbalanced and looks at a 8 year period.
Lets say I introduce a new variable, that might have only one value or none for a certain country.
Stata seems to completely eliminate that country from my regression - since the number of goups is decreasing.
I cant find any information about the minimum amount of data points, that are necessary to keep a country from beeing excluded.
I expected stata to just exclude a country from the calculation of a certain coefficient - if insufficient data is available, not to completely leave it out.
If anyone could provide me with further informations about this topic!

Thanks in advance
till