Cat6 jacks are slightly different in their layout and crimping.
They’re 100% compatible with all your existing ports, however they do require a cat6 compatible crimper which crimps slightly deeper than on cat5.
So likely you’re having trouble because the pins are not deep enough and are preventing the jack from being insertable.
For me it’s either 600m to a small corner shop or about 1.1km to a larger store.
I do 3km periodically for another store, but I wouldn’t do it in 30c weather. I’m barely willing to exercise in 25c weather.