During the data collection of the DHS survey, some questions can be missing such as the date of birth but some other questions can't be missing such as the marital status since these questions are used during the skip to define the base of individual to ask certain questions to. The respondent should be able to know whether he/she is in union or not. The questions where we don't allow missing values can be sometimes by accident be missing but these are very rare. To be able to identify which ones for which we allow missing and those for which don't allow this value, you need to look at the map distributed with the data files

In case the number of missing is considerable the one sure thing to check is whether the question is asked to the entire population or to a subset. In all cases, you need to check the base population to which the question is addressed. On the top of my head, we can't ask about "knowledge about condoms to reduce the chance of getting the AIDS virus" unless the question to which the person interviewed has a previous knowledge of the AIDS virus. This information is given, in general, in the DHS Recode Manual as the BASE of the question. The BASE is not given in all cases and you will have to review the skip pattern used for the survey in question.