The sample reduces when running logistic regressions

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The sample reduces when running logistic regressions

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I am running a logistic regression on different samples using multiple imputations:

mi estimate, or post: logit Y X1 X2, vce(cluster PSU)

I have run the regression for the first samples and Stata gave me back results where all the observations are included, except for one sample for which I have got 1332 observations but 1292 observations were taken by Stata for the regression. In addition, the categorical variable education (i.education which takes values 0, 1, 2, and 3) presents a problem: 0.education is the baseline, for 1.education and 2.education Stata gives me back the relative coefficient, whereas 3.education is omitted although there are 168 observations.

Which could be the causes of these two problems?

Thanks in advance.