ask: no aid dummy in panel data using stata

Dear all,

I would like to ask the utilization of dummy variable in stata.
Well, here is my model
y=beta1 lnx1(1,x1)+beta2 nox1+beta3 lnx2 (1,x2)+beta4 nox2

so basically x1 and x2 are type of aid which given to countries. it is panel data 6 years long with 20 countries.
i use the model because there are alot of zero aid. no aid dummy used to avoid ln 0. since ln 0 is 1.
however when i run it in stata using Fixed Effect (i used fixed effect in my analysis) the result is strange. the nox2 (one of the no aid dummy) is omitted from the result. i dont know why.
please somebody please explain and if possible the solution. thank you!