After 250 miles on a new bike, the cables don't have the same tension in them. The cable housings settle a little bit into the ferrules and frame, giving the cables more slack. Some say the cables stretch, but I think it's just an easy way to say it.

It could be that your front derailleur now rubs the chain. Or your chain is touching the next cog in back. Either way, all you have to do is adjust your cable tension. Try a quarter turn, loosening them like your unscrewing something. If that doesn't do it, do another quarter turn.

There's a barrel adjuster on your rear derailleur right where the cable housing meets it. You might not have an adjuster for the front derailleur, but you probably do. It might be on your frame, or part of the cable housing under the handlebars.

This is the solution for more than 90 percent of new bikes that have developed shifting problems after the normal break in period of around 200 miles.