For venting problems there is ususally 3 different problems.
The toilet will flush slowly, air gurgling or the water will syphon from the bowl.
You may have a vent problem but not at the top of the stock.
If you fill a basin in the same bathroom and let the water out, you should see it go down slowly.
If the shower and toilet is one the same run it may be a problem between the unit and the stack.