Occasional Shifting Problem

Not sure what bike you have but have you checked the shift linkages everything tight at the shift lever ends ( connection to splined shaft at gearbox and shift lever ) and any ball joints clean & lubricated

 

Not sure what the problem is, however, many years ago and many bikes ago, I learned to preload the shifter going up in gear. This has stood me well on every bike and brand. Some bikes just don't like banging up a gear.

 

Drive chain too tight or too loose? Tight spot in chain?

 

Sometimes an oil change will improve things.

 

if bike spends a lot of time on sidestand this may have some bearing on this,have bike more vertical on a stand or similar when you park it up between uses,and as already mentioned an oil change could well help.