How to buy ticket on KM?

I have the KM app. Whenever I fire it up I'm not able to find the route I'd like to take. Today I'm at Aleje Jerozolimskie ready to take the R80 that leaves in 2 min. I open the app, write in the departure station and arrival, but it only shows me the next departure in 30 min, a ticket that won't be valid for my ride. I've just been buying this one for all my rides. The other day I was checked by a ticket guy and he said I had to buy a new from him because it's not valid. He didn't speak english so I couldn't get him to explain what I'm doing wrong. Anyone, please?