Three things decide how a charango plays, and each is a consequence of the instrument being small, nylon-strung and struck all-at-once rather than plucked string by string.
The nut, first and always
Five courses across a very narrow nut leaves almost no margin. Each pair wants its two slots at the same depth so both strings speak together under a stroke, and the gaps between courses want to be even so the hand does not have to aim. If one string of a course sounds late or dead in a full stroke, look at its slot before you look at anything else — a slot cut a fraction deep leaves that string sitting lower and getting struck differently from its partner.
Action for a stroke, not a pluck
The figures on the rasgueo pages move the whole hand across all five courses, so action is set for that rather than for single notes. Low enough that a fast stroke is not work; high enough that nylon, which moves in a wider arc than steel, does not slap the frets under a hard one. Nylon buzz sounds duller than steel buzz and is easy to miss in a room — check it quietly.
Humidity is the real maintenance
A small carved body in a dry winter is the classic way to lose a charango. The instrument comes from high, dry country but it was built in the humidity it lives in, and a heated room in another hemisphere is drier than anything it was made for. A case and a humidifier cost less than one crack repair. Heat in a car is the other reliable way to destroy one; there is no cheap version of that repair either.
Tuning ten strings is not a fault
Ten strings, five of them paired, on a short scale in nylon: this instrument goes out of tune more than a guitar does, and it always will. Tune the two strings of each course to each other first, then the courses to the instrument, and keep the tuner nearby rather than treating each session's drift as a sign of something wrong.