Probably the issue comes because you did not generate the missing weeks.
1.Ideally you use a master calendar or qlik components to fill in the gaps.
They use autogenerate to fill in the gaps.
2.If the blanks are in your dataset you can use Isnull or trim to identify them and create a field WEEK_CNT to count them.
I'd agree with Dion that you may want to use a Master Calendar. If you choose to use 52, then you may be limiting the analysis to yearly for when users makes different selections... but any way if it works for you it works!
Just in case it helps, I created a process to automatically create a master calendar with few parameters: https://community.qlik.com/docs/DOC-19202