I am working on a standard gravity model and am running a cross-section and panel regression which include fixed effects (country fixed effects for cross-section and country-time and country pair for panel). The results however omit some variables which are not "fixed" across time or country. There are 21 countries in the database. Any reason why that is so?
Edit: Update: I tried various combinations of exporter, importer fixed effects and sort of got through the problem of collinearity (I am using PPML command). However, some of my coefficients are as high as 43 now which is ridiculous.