After watching again I can understand why Spourt and Pomfrey were absent because as it is McGonagall only had two screens in where she had lines and Snape only had four screens where he had lines.
So even if Sprout and Pomfrey were in movie they would just be there as extras and would be lucky to even had one line and Gemma Jones and Miriam Margolyes might of be asked back but once they were told there roles would be no more then standing extras they decided better of doing other roles where they get lines.
There are many actors (of all ages) who appear in most if not all the movies of the series. They're not always relevant to the plot (they're even used as glorified extras most of the time), but it keeps the continuity.