The Koch Brothers have not released the usual 'promised' four hundred million dollars* to any Republican candidate. Why not?

The Koch Brother always donate their pocket change to the presidential race every four-plus years. A usual primary amount of four hundred million has still not been granted to any candidate. (* amounts vary as to the actual dollar amount beginning donated- latest NYTimes report doubles this amount for total amounts the Koch’s donate.)
Why are they holding back? Do they want the Donald to fund his own fun?
Are they silently commenting upon the candidates running this Election?
Will allowing Donald, while not ideal, yet who is Big Business, like them win without donation nor comment signal a quiet Yes? So his bid might have some positives for their own interests.
Neither an endorsement nor a donation.
Remember, the Kochs have lost a great deal of their political donation monies on the unsuccessful McCain and Romney campaigns the last two elections. Gun shy without the NRA? (a joke, a jest- just making sure… )
What’s up with the Kochs?