Starts voice does not always work

Quite often when I select ‘starts voice’ for a note the preceding rests Dorico has automatically put in do not get removed, which I thought is what this does.

What are the conditions for ‘starts voice’ to take effect?

See a previous primer on primary vs secondary voices: Unasked for flipping of notes - #23 by pianoleo

If it’s a secondary voice and the voice has automatically been stopped more than one bar before the current entry, Starts Voice should successfully hide the padding rests at the start of that instance of the voice, e.g. it’ll remove the rests at the start of that bar. If it’s the primary voice or the voice is continuing from the previous bar - it hasn’t automatically stopped - you’ll also need Ends Voice on the previous note. Using Edit > Remove Rests sets both of these properties at once (and if you filter the relevant voice first, either by way of a custom shortcut or via the Jump bar it’s quicker too).