Some of the columns are missing at lower level even if they have exist in Staad model
This is a typical case for few cases where some columns has intermediate nodes between levels and some re not. For example the support level for all columns are at 0m level. The next floor is at 3m. So, it is expected that all columns will be designed from 0 to 3m for that story. But if for few columns we have a node due to some mid landing beams (or any other reason) there may be a difference in representation. For the columns with intermediate nodes, it will show 0 to 1.5, 1.5 to3 m. But for the same story, for other columns without any intermediate nodes it will show 1.5 to 3m in design output. Though all the calculations for those columns will be based to 3m height (0 to 3m) and results will be correct. This can also be checked by generating column elevation where you will find the correct drawing and also in the detailed calculation the length considered is correct.
However, there is a concept of merge and demerge levels in RCDC using which you can dix this issue. Please refer attached document from help file to know more about merge and demerge level and their implication in design process.