Two issues here: firstly, DSS was brought up in the knowledge that his dad would do ANYTHING to keep him on side, but nothing was ever expected in return. I totally agree with this Petal, despite my DP doing everything he can, including dropping off and picking up DSD who is 19 and at Uni (and lives with her Mum) - they don't bother coming to see him at all anymore. Sometimes they indicate that it is to do with me, as I dare to ask them to say Hello and occasionally ask them to have the living room for my TV and not the computer, which 'makes it awkward'. They are not bad girls, lots of good qualities, but I've seen them become indifferent and selfish to DP. Their Mum kicks off quite a bit, which isn't exactly secure for them, but they noticeably respect her more, and she also demands them around when she is lonely, and so they do.
Secondly: we had a ridiculously strict access rota, maintained right up to uni, so their relationship/contact was never based on free will, so now there's no rota in place to structure contact, it simply doesn't take place. I feel differently about this, I asked for a (moderate and not rigid) rota to be put in place which did really help for a while. Before the kids were being sent over to our house whenever their mum wanted them out, with no communication at all. It wasn't their freewill, or when it was, it was to use our XBox game - and if DP tried anything but Disney parenting, they'd troop on back to their mums.
No expectation that it was about keeping up a relationship. Now there is no 'rota' - the kids have decided not to come around at all - partly as they are older, and their mum lets them have the house while she is away at weekends.