Proto-Norse vowels in ultimate

Hi!

I'm currently gathering data for my new long-term project S17, which
will be Latin with Icelandic sound changes.  The conhistory will
probably start around 150~200 AD.  I will probably base the conlang on
the Classical Latin vowel system because it particularly fits the
Proto-Norse vowel inventory better than Late/Vulgar Latin.  The time
frame should be ok for this, but the details have not been worked out
since I'm still learning.

I plan to abuse selectively a few of the Proto-Germanic (PG) >
Proto-Norse (PN) changes to adjust the sounds of Latin to what I need
to run my Grand Master Plan.  Yesterday, I ran into a yet-unanswered
(to me) problem in the Proto-Norse sound system:

I gathered that the possible vowels in the ultimate of Proto-Norse are
the following:

Short:  /a/,
        /e/,
        /i/,
        /u/

Long:   /&:/ (_á_),
        /a:/ (in -_áR_ from PG -_ai~z_),
        /e:/,
        /i:/,
        /o:/,
        /y:/ _iu_

I.e., from a symmetric system, /o/ and /u:/ are missing.

PN does have /u:/, but not in the ultimate according to my data.

My question is: why not?

I understand that there is no /o/ due the shift of IE /o/ > PG /a/
(very few /o/ do occur but they are seldom and not in the ultimate),
and all the others I also understand, but my sources mentioned no
sound shift(s) that would remove a long /u:/ from the ultimate.  Did I
oversee anything?  I mean, it may well just be defective accidentally,
but maybe there's an obvious reason.

If there really is no ultimate /u:/, I will have to think about the
Latin u-declension.  There are several alternatives, but I first
wanted to check out whether my phoneme lists are correct.

Bye,
  Henrik