Proto-Norse question

Hi!

I just stumbled over another phenomenon in Old Norse (ON) that I don't
fully understand.

I read about the rule that in early Proto-Norse (PN), after light
syllable, -j- in -ja- remained a glide, while after heavy syllable, it
became a syllabic -i-.  Example:

   Glide: PN *niðjaR   (ni.ðjaR)    ('ni-'  is light)  (> ON niðr)
   Vowel: PN *herdiaR  (her.ði.aR)  ('her-' is heavy)  (> ON hirðir)

But now I read that there is *þunkjan in PN ('to think'), while I had
expected *þunkian by the above rule, since the first syllable is
heavy.  But still, the word is þykkja in ON and not *þykka, so it was
not trisyllabic in PN.

My explanation is that the above heavy/light rule does not hold after
velar plosives, which got palatalised by the -j- and thus the -ia-
could not become disyllabic.  However, I have not yet read this
anywhere.  Further, I thought the palatalisation happened later than
the above j > i shift, so how could it influence it?  There have
definitely been some palatal effects quite early though, i.e. the
gemination of -kj- to -kkj- and -Gj- to -ggj-, so maybe this
explanation is still feasible.

Would you think that this guess is right, then?

Bye,
  Henrik

PS: Due to lack of more books, I currently use two sources:
    - A. Zal-uska-Strömberg, _Grammatik des Altisländischen_
    - Tor Gjerde, _Norroen Dyrd_ (http://http://norse.ulver.com/)
      (This is in Russian, which makes it hard for me to read,
      but I have a dictionary and a lot of time...)