technique with a 64

I am trying to decide whether to send a second 64 back to Hohner (in one week). The first one had no response on the #4 blow on the low octave with the slide in - got replaced in two days. The second one has lots of "rattle" on the low end and some "buzzing" also. This is my first experience with the 64 (limited chromatic experience) and I thought that this might be more my issue than Hohner, although I have read some similar complaints on other forums.
Does this sound more like just an inexperienced player issue, or bad luck on two rather expensive harps.