Slade Green to Brighton

Hi everyone,
I think I'm going crazy but I can't seem to get a sensible itinerary for a trip I am planning with my family.

Outward from Slade Green at around 9am on Saturday 25th May and returning around 5pm on Sunday 26th May.
2 adults and 1 child with a Family and Friends railcard.

Both Southeastern and Southern's sites give me outbound trips with no issue but only one return choice all day and thats at 21:42 from Brighton - can't get any earlier journeys for some reason.
Just for comparison I tried RSH to see if that worked and that won't give me any journeys for either outbound or return!!! (not that i'd buy tickets with a booking fee anyway)

Any one know why this is?
Thankyou
Blakey

There's engineering work closing the Woolwich line all weekend. Whilst there might be Sidcup rounders from Plumstead on the Saturday, it looks like you might have to travel via Dartford on the Sunday. Not sure if this is the reason, but it might be connected. The RRBs start at Lewisham and go via New Cross then Deptford to Plumstead.

Engineering work involving a rail replacement bus between Lewisham and Plumstead means that the quickest route from London Terminals to Slade Green is via Sidcup and Dartford, but that's not a permitted route. The only permitted route that doesn't involve a replacement bus is via Bexleyheath and Dartford, but that has very sub-optimal connections off the trains arriving from Brighton (typically a half-hour wait at Victoria), so some journey planners (e.g. SilverRail which powers National Rail Enquiries as well as Southern and Southeastern) aren't showing it either.

So really, this is a journey planner bug. Use a site that uses a better quality journey planner, e.g. https://trainscanbecheaper.info/ which uses FastJP, and you will see a good selection of itineraries (see attached screenshot for an example).

EDIT: MikeWh got there before me, but at least we have come up with the same explanation!

Another option to buy the ticket would be to use the c2c website which seemingly ignores the routeing guide and allows all the itineraries via Sidcup anyway