Hi Plummet,
sounds strange but if it is a combination of many things it could be possible:
1. as joriws said: if you are on the boarder to go upwind it is nothing about speed but all about angle so you have to lower the speed (as long you have still a harmonic current around your foil) to normally around 25 km/h to improve the angle towards the wind. This is just a clear geometric principle. So maybe you really have been to fast
2. there are spots and situations where you don't have to much current close to the beach where you better see it but heavily outside. Maybe this was the case, but could have been seen if you make a constant bearing when beeing outside watching the beach while sitting on the board.
3. and 4.
By all respect to your self build foil. It can be that the foil and also the foil kite have just a very low angle of glide due to design or use. I have driven low performance foils as the first Liquid Force and also no strut kites with very low angle of glide.
Happily never combined both, but if I would have done, combined with point 1 and 2 I could imagine to be able to ride but not upwind.
If your upwind tracks under good conditions don't look close to this combination of an very high performance foil with a pretty low performance kite (Wave Kite with AR 3,5) in not perfect conditions:
Then it can be worth to check your foil and your Chrono:
If the bridle of Chrono has seen much use and was folded to often it can be so much out of shape that the original high performance of the kite is lost, try to check the mixer, and if you are not affraid of the effort do a "long mixer test" or give it to someone who knows the kite well in good condition.
Good luck