If a child is unhappy about anything with the parent, then in most cases that should be sorted out with that parent.
I totally agree with this statement, but my experience is that it often doesn't happen. A common theme as to why kids don't want to go to see their dad any longer is that they don't feel part of the family, often because they are expected to follow the dynamism of the established flow, which doesn't fit at all with what they would like to do, or/and, they get very little time, or none at all with their dad. It becomes a 'once the point'. If they try to broach the matter and are told that they are being selfish because life can't change completely to suit them when they come, then that's often when they stop wanting to go all together.
My children still go to their dad, but mainly as a sense of duty and to see their sister. They get very bored when they are there because nothing much happens. They are expected to babysit, participate with the chores, but it's extremely rare that they go out and do anything. Their dad is often tired and wants to rest and they don't have much money to do anything. DD use that time to do a lot of her homework, DS spends most of the time watching TV or on his phone.
Thankfully for them, their dad is not putting pressure on them to go every other week-ends and is quite happy to be flexible with their visits. This has taken pressure of them and as a result, they don't dread going. I am pretty sure that if their dad had insisted that they go every other week-end without fail, regardless of what happened in their lives, they would have made the decision to stop going at all.
o the 3 of the are going to have a talk and try to get to the route of the issue, if there is one ....
That's good, hopefully it will be resolved positively. Why are you doubting there is a reason for him not coming? It might not be a valid one in your OH's eyes, but there's got to be one valid enough for him to have made that decision.