Welcome to the site !!
When you inspected the right side , were you doing it while shifting ?? , to see if everything was functioning , or did you just note that everything looked in place ??
It is possible that the shaft is bent , and/or the shift drum was damaged (cases need to be taken apart) and/or a shift fork could be bent (again , complete teardown of engine to check this) , have you tried to shift the bike while its on the stand by rocking the rear tire while shifting with your hand and noting whether it is binding then ???
Some more background on this is needed to even come close to trying to answer ..... First , did you just buy the bike ??? , did you recently crash , specifically on the shifter side ??? , were you riding it and it just started doing this out of the blue ?? , or , did you land (flat land) from a jump and jam the shifter into a lower gear with your foot ??? .... There are many variables that could cause the symptoms you are describing , and this info might lead us to the answer ......