I'm currently working on a chart - Snare, Tenors, Bass, Cym, Timp, Glock, Xylo Vibe, Marimba - and just loaded up the VDL 2.5 sounds.  All of the instruments play back correctly when the Kontact program is set up except the marimba, loaded on Channel 10 (Skipped a channel due to an Aux part that might be coming)

All instruments playback except the Marimba.  Did I miss something?  Mind you, this is still using the VDL2 template, not hte VDL 2.5, but will that really cause that much of an issue?

Have you tried a different channel? Might be a conflict.

Not sure if you're using Finale or Sibelius, but be sure in the notation program you've properly set the output channel for that staff to be the same as the input channel the marimba is set to in Kontakt Player.