Ovid's Metamorphoses 7.66, here I marked my attempt:
nempĕ tĕ/nens, quŏd ă/mo, grĕmĭ/oqu(e) in/ Iasŏnĭ/s haerens
That makes the 3 first feet dactyls and the fourth one a spondee, but the answer from hexameter.io says the fourth is yet another dactyl (and also mentions there is an elision, so I probably got the elision right; unfortunately I can't see the correct scan itself). The i in qu-in is parsed long, because the I of Iasonis is a consonant. I can't figure out what I missed.
As often with tricky verses, the key is in the name. The only way I can scan that right is reading the name as Ĭ-ā-sŏ-nĭs. The initial I/J of a name can easily vary between vowel and consonant in poetry, but in this case Lewis and Short do give precisely the same reading I got.
The first syllable of Iason goes into the fourth foot and makes it a dactyl instead of a spondee. Your reading does scan properly and it would not be an unusual licence, but it seems that the canonical reading is with the vowel I instead of the consonant J.
Just to make it clear what Joonas said explained in his post, here is the line scanned out:
nempĕ tĕ/nens, quŏd ă/mo || grĕmĭ/oqu' in ĭ/asŏnĭ/s haerens
The reason the initial letter of Iason is a vowel is because it's not Latin at all: it's Greek, from Ἰάσων, and ancient Greek did not have a separate [j] phoneme (though Allen says that it was likely pronounced as a glide following the iota before another vowel).
So in words where we might expect a J in Latin, in Greek it's always vocalic. In Latin, you'll find the odd exception, but the general rule is that it's a poetry and most often scans like one (so Iacchus and Iapyx are trisyllabic and Iocasta and Iasius are quadrisyllabic).
