A malfunction has been detected , have it checked out
Main reasons are low on fluid, and the bands are weak or bad.
when your gears are grinding, and u cant change gears properly. also when your flooring it and it sorta slips instead of ingaging properly.
Matching the gears to mesh properly.
To properly change gears on a bike, use the gear shifters to move the chain onto a different gear. Shift to a lower gear when going uphill or starting from a stop, and shift to a higher gear when going downhill or riding at a faster pace on flat terrain. Make sure to pedal lightly while shifting to avoid damaging the gears.
the clutch is either worn, or needs adjustment if clutch is not the problemthe syncros on gears may be worn other than that, maybe driver error/are you dropping reves enough to change gears properly
Unfortunately, you can't change gears on bikes in HeartGold.
You need to be taught to do this properly and you need a lot of practice. Basically you change up to go faster, but sometimes you change down for acceleration or for safety.
bad linkage in shifting cable
Your bike gears may not be shifting properly due to issues such as cable tension, derailleur alignment, or worn out components. It is recommended to have a professional bike mechanic inspect and adjust your gears to ensure they function correctly.
To fix bicycle gears that are not shifting properly, you can try adjusting the tension of the gear cable, checking for any debris or dirt in the gears, and ensuring that the derailleur is properly aligned. If these steps do not work, it may be best to take your bike to a professional for further inspection and repair.
You have to change the gears
It can be the clutch, it doesn't work properly. And check the transmission fluid level. Sad news is that if it's the clutch you will have to replace it as soon as possible.