DNA - no match?

[ElizabethHerts]

There is an Ancestry tree with one of my Cornish sets of 4x-great-grandparents shown as ancestors to the tree owner.

I can see the tree owner descends through one of the daughters, sister to my 3x-great-grandmother. The research looks sound.

The tree owner has done a DNA test but we have no match.
Should I expect a match if our research is correct, or does this sometimes happen? Should I suspect that somewhere a parent is different from my or his research?