What could be the issue if a mountain bike is not shifting correctly and if you got a new bike that isn't working, replaced it with the same model of bike and just got one that has the same issues, should you just get a new bike?

The mountain bike is an old specialized bike.

The other one is a road bike that is called coop ARD 1.2 which from my understanding is one that REI makes. I have had two of the road bikes with similar issues and someone else had similar issues with it. I don't know if it was just someone put these specific bikes together wrong around the same time and we just got unlucky to get those bikes that were put together wrong, but it seems strange to me that there are at least three of the same bikes having the same issues from the same location of REI which is why I am wondering if I should just take it back and get a different type of bike from some other place especially given how no one could figure out the issue no matter how long they had to work on it.

/r/bicycling Thread Parent