I am afraid that there is simply no way that you can force your ex to have the children more often than he wishes.
I know that this is not fair - especially when there is already a set pattern that the children are used to
However if he will not stick to the pattern then a new one needs to be agreed - and if he has the children less he will have to pay more Child Maintenance
You will need to try and negotiate with him using Family Mediation to agree a new pattern.
If he has them from Friday to Monday alternate weekends plus one night in the week then tat would be five nights a fortnight
With regard to the money it is not too late to make a financial claim - but it is more likely that you will get a lump sum in lieu of a pension share since a Pension Sharing Order can usually only be made with in a short time of the Decree Absolute.
I am sorry to have to give such bad news - please ask if you need further details