I have a 2007 Honda Accord 4 cyc automatic that sometimes has a problem shifting into a higher gear. The car has 34K miles and it always goes to the dealer for routine maintenance.
I noticed the problem today in Boston on 90 when I was trying to get out of the city traveling between 70-75 and the car didn’t want to shift.
As long as your Powertrain Warranty is still in effect, you do not need to try to diagnose the problem. Take the car to the dealer’s service department, describe the symptoms as completely as you can, and let them figure the source of the problem. In fact, even if you do volunteer your diagnosis, they will ignore it as they are required to go through a standardized diagnostic protocol for warranty claims.