I Need Major Bathroom Remodeling Help. My Contractors Screwed Up The Job :(?

I hired a company thru Service Magic to remodel my hall bathroom. I had them tear down the walls and take out all of the existing tiles. I purchased marble tiles for the shower and glass tiles to be used as a border. After they “completed” the job I took a look at what they installed.
The border tile goes in and out. So if you slide your hand across it, your hand goes in a wave motion. I”m trying to explain this to the contractors and they are telling me that all glass tiles are installed this way…??? They are saying because it’s flexible it causes it to go in and out. Please help me.
Why did this happen. What can I say to them to make them understand that it was not installed properly?