Even when the connections do work, journey planners don't always show them.
I first met this a few years ago, planning a Sunday trip from Surrey to mid Wales, which involved an RRB from Blackwater (on the North Downs line) to Reading. The NR journey planner told me I couldn't do it, as the first train from my local station wouldn't let me get to Swansea in time for the last Heart of Wales line train.
When I investigated further, I found that the nominal connection time from train to bus at Blackwater was 7 minutes, no problem as it's only a short walk. But because that's less than the journey planner's minimum, it instead showed a connection onto the next bus, an hour and seven minutes later.
As it turned out, there was no risk of the connection not working, to their credit GWR had staff at Blackwater directing passengers and making sure everyone was on board before the buses left. But it seems the minimum connection time often isn't (or can't be?) over-ridden, as I've encountered the same issue on occasions since.