The fact "one or two spokes are getting loose" may be two factors. You could mark the ultra-loose spokes with tape to see if it's always the same spokes that are loose.

- Those nipples have stripped treads. Replace these nipples and you should be OK.

- Not enough tension in the spokes. Undertensioned wheels tend to loosen very often. Do as usual and straighten the wheel; then turn all nipples by 1/2 turn or 1 full turn.

- Twisted spokes. Sometimes, when you screw on a nipple "just a bit", you hear a "ping" and the spoke turns with the nipple. So you are actually twisting the spoke rather than tighten it... and the wheel will remain true until your next bump. Keep two fingers on the spoke itself as you tighten its nipple to make sure the spoke doesn't turn. When you tighten by, say 1/2 turn, it's better to turn by 3/4 turn and turn back 1/4 turn after.