Hope this helps ....
Most probably hindi pareho ang framerate nang Video Stream & nung Subs file. To check, you can use MEDIA INFO then use SUBTITLE WORKSHOP to change the subs framerate (if needed).
Also, you might be synching video stream with a "different' subs file e.g. Theatrical Cut versus Directors Cut.
Now if above has been done & its still out-of-synch ..... I would say that issue is not with your NMT units. I am using A110 (have also used HDX before) and have never encountered issues with subs before.
I have encountered something similar before - but it was video stream related ..... more specifically, the HDTV ripped movies are the hardest to synch with.
By the way, what specific movie do you have problem with? I probably might help you more on this.