I've been trying to scan part of Ovid Metamorphoses VIII, and have noticed that sometimes the word "tibi" seems to need a short second syllable, e.g.
line 94) sed patrium tibi crede caput!" scelerataque dextra
line 131) officium tibi sit! te vere coniuge digna est,
And sometimes it seems to need a long one, e.g.
line 91) Scylla tibi trado patriaeque meosque penates;
line 98) orbe suo, tellusque tibi pontusque negetur!
line 137) praeposuisse tibi: tu plus feritatis habebas.
I suppose that lines 91 and 94 are covered by the rule that allows the syllable preceding cr/tr etc. to be either long or short, but what about lines 131 and 98 and 137? In line 131 I have "tibi" as the second and third syllables of a dactyl, whereas in lines 98 and 137, I have it split between two feet, with the second "i" in the first syllable of a spondee. Have I scanned them wrongly? Is there a rule I'm unaware of, or can that second "i" in "tibi" (and presumably therefore, "mihi" and "sibi") always be scanned however is convenient to fit in with the rest of the line?
Thanks for any advice you can offer,