The moving parts
Royal Mail needs three things to match before it will let you change delivery: the tracking number, the delivery postcode the sender used, and the parcel actually being in their network. If any of those are off, the option simply doesn't show.
The attempt limit catches people out. Enter the wrong postcode a handful of times and Royal Mail locks the redirect on that parcel. It's protection against strangers redirecting your post, but it means a typo in your address book costs you the option entirely.
Timing on drop weeks
Royal Mail's window tends to be more forgiving than DPD's, often the whole evening and sometimes the next morning. The catch is the start: the option opens when the parcel is scanned at the first sorting point, which for a retailer using a collection service can be late in the evening.
That means the reliable time to redirect is not when the dispatch email arrives, and not first thing next morning either. It's a window in between that you have to be awake for.
Doing it without babysitting
The same principle as any carrier: save the shop, save the postcode against the parcel from the confirmation email so it's never retyped, and have something check for you until the option opens.
For Minted members this is automatic. Parcels come in from your inbox with the right postcode attached, you pick a Parcel Shop once per address, and the redirect is booked the moment Royal Mail opens it. The confirmation lands in your email and the parcel is marked redirected on your tracker.