Hi Jamie
I have read that snapping to feature services is turned off by default as
it can slow down projects a lot. So you need to enable snapping on feature
services.
Esri wrote
By default, snapping is turned off for feature service layers to improve
map performance. To turn snapping on, in the Contents pane, click List by
Snapping , and check the check box for the feature
layer.
Regards
Mervyn