There are many reports of people having the Nero trigger work on Canon, Nikon, etc... but their Pentax cameras were intermittent.
At some point I was looking into getting the Nero trigger, and I was sure that their own web page listed that it was some what unreliable on Pentax cameras. (cannot find that now, of course) :-(
I have seen such issues mentioned with other remote triggering devices as well, so is probably not limited to Nero.
I have not found anything that leads to why that would be a problem, but it is apparently not uncommon.
My uneducated guess is that Pentax requires a longer trigger time than some other brands. Historically a human is pressing a button and that is a huge length of time compared to a device that should be able to trigger 7 times a second.
Dunno why the K5 would behave differently, except that every time I read comments on the issue the words "intermittent" or "sometimes" are used. Maybe just lucky/unlucky??
FYI,
http://www.ephotozine.com/article/nero-trigger-review-21405 https://www.pentaxforums.com/forums/22-pentax-camera-field-accessories/237798-nero-trigger.html