Have you tried a different insole? Sometimes that's the difference. Zamberlans are by far the best fit for my narrow heels, but I run either Sole insoles or orange Superfeet in mineRelative to every other boot you've worn before, did the 980's you settled on end up being a smaller size? If so, how much smaller?
I typically wear a comically small 8.5 in everything. I found the 980s on sale before Thanksgiving and got a pair in 8.5. Heel slip.
Sent those back and got a pair of 981s in 8. Had heel slip. According to Zamberlans site, the 981 fits more true to size and they don't recommend dropping a half size from normal.
Sent those back for a refund and found a pair of 980s in a size 8 on clearance with Midway. Got them in today. Heel slip. Not as much as the other pairs, but is noticeable enough I'm leery of them.
I'm starting to wonder if Zamberlan is going to work at all for me.
Other than the heel slip, the fit in the forefoot is great. Snug, but not tight. Excellent boot.
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