It is very difficult to determine if your ancestors are in fact missing from the 1926 Census, or if you just haven't been able to find them.
Note specifically the fields that the 1926 Prairie Province Census allows you to search:
This is actually one of the richer searches you'll find for any set of records anywhere.
Surname: Try searching all possible spellings. Look at your other
records and think how they may be misinterpreted. Take advantage of
the wildcard ability.
Given Names: Generally, leave this blank (or use wildcards), or try
just a given name and no surname.
The Age is one way you can really narrow down people. Do you know
the age your gg-parents would have been in 1926? Even if you think
you do, try one and two years older and younger as well.
Province: In your case, always specify Saskatchewan.
You should also look for the Uncle they were joining. It looks from your Passenger Declaration to be "Anton Gittel" or something similar. He was in Viscount, Saskatchewan.
Look through the Districts and Sub-districts list to find Viscount and you'll find it in District 19 - Humboldt, Sub-districts 27, 28 and 29.
Use the district number in the custom search to look for the uncle and his family and maybe your grandparents as well.
Bruno is also in District 19 - Humboldt, but Sub-districts 55 and 56.
You should be able to find death records, cemetery burial information and/or other information that might give you additional clues as to why you can't find them in the 1926 Census. Dave Obee has an excellent guide for Canadian Genealogy that includes Saskatchewan resources.