Changing the rim strip was a good move by the LBS, but it does not guarantee against other type punctures. You need to find the hole in the tube and realign it with it's position in the tire. This will point to the spot in the tire or rim that caused the puncture. That's why it is a good idea to always mount your tires with the label centered at the valve hole.
Take some cotton, rub it around the inside of your tire, it will snag on any tiny sharp object that the eye misses.
It could be a sharp edge at the valve hole. Feel around the hole for any sharp edge. Gently touch up with a rat tail file.
It is important to know if these flats are all caused by the same object, otherwise you are just shooting in the dark hoping to get lucky.