Tableau applies filters at the database, before the calculations in the worksheet such as reference lines. So when you filter a group of shops, the view no longer has this data.
There are three workarounds --- See Maintain percent of total in Tableau when using filters
If you just have one or two filters, Option 1 / table calc filters are probably the way to go. They are much easier than they sound, and there is a more detailed description here: A Jedi (Filter and Table Calc) Trick | Tableau Software
If you have a bunch of filters, Option 2 (or Option 3, if you know you'll never need an extract) are maybe a bit quicker / easier.
Whatever you choose, you'll need to create a calculated field for this value and add it to the level of detail shelf. Then you can use this field/value for your reference line.