I am trying to estimate the gravity model of international trade using Fixed Effect Model. I have data from 1970-75-80-85-90-95-2000-05-10. I used the code

xtset pairid year where pairid is the alphanumeric pairid for each trading partners. Then, I used reg xtreg ln(Trade) ln(Dist) ln(ImpGDP) ln(ExpGDP) i.year, fe.

In the result, ln(Dist) was removed due to collinearity issue. I am actually interested in this particular coefficient but stata is omitting it. Is there something that I did wrong?