I have a young adult daughter. She is away at uni so we don't see her much during term time except for the odd visit to her uni town or an occasional weekend at home for her. However, we do spend a good amount of time with her in the uni holidays, we speak to her on the phone quite regularly and we text a lot in between. She also spends plenty of time with friends and with her boyfriend, time doing hobbies and of course working. I love every moment that I have with her, but I am genuinely happy for her that she has a healthy balance across the many different areas of her life.
The key thing is, I still feel very close to her while recognising that we don't have to see each other or be in constant contact for our closeness to be maintained. It is more about the quality of our interactions than the quantity. And I do think the onus is really on the parents of adult children to make contact work for their children.
You need to recognise that, while they will always be your children, they are actually adults now and their autonomy/independence should be always be respected.
You need to understand that they are busy building their own lives, and much as they may love their parents, they will have their own priorities to pursue and their own relationships to maintain - so they should never be made to feel guilty about doing things that don't involve you.
You need to ensure that the time that they spend with you is actually enjoyable and rewarding for them, so they they spend time with you because they want to, and not just because they feel obliged - they should ideally come away from their time with you feeling better about themselves, and not judged/criticised/guilty of disappointing you. You should be there to support them and cheer them on, not drag them down.
You should take a genuine interest in their lives and in what they want to share with you, while respecting the fact that there may be things that they don't want to share or discuss. And that's ok, their lives are not your property.
You should make it clear how much joy they bring you, but you should also invest heavily in your own life and relationships so that they don't feel responsible for your happiness - that is too much of a burden for a young person to carry.
You should offer practical help when asked, if you are willing and able to do so, but you should also respect the fact that they are capable adults who might have their own way of doing things which isn't the same as yours. And that's ok, because they are people in their own right and not just extensions of you.
If what you add to your adult children's lives is positive, supportive and respectful, and they feel good when they spend time with you, then they will naturally want to maintain the relationship and they will make a genuine effort to do so. If you are critical, demanding or judgmental, if you drain their energy, make them feel guilty or otherwise make them feel shit about themselves, then it's likely that they will choose to distance themselves.
The power is in your hands.
ETA This adult child in her mid fifties went from seeing parents once every 12-18 months when living abroad to now seeing my elderly father every single day - because they respected what I needed from them in those younger years, I am now happy to do what he needs now in his old age.