The Polo suffered further rear differential and handbrake issues on last week's Rally Argentina.
A revised specification of handbrake operation - hydraulic instead of pneumatic – was running on Andreas Mikkelsen's third car, but it will not be included on the points-scoring Polos until it is absolutely ready.
Sebastien Ogier and Jari-Matti Latvala both suffered problems with the part and were vocal in their desire to see the upgrade out in time for the Acropolis Rally at the end of the month.
"I know that the team is working hard on it, but still there are a few problems on the new system we want to homologate," Ogier said.
"Normally we will have this for the next rally, but already I heard that for many months... It's a shame.
"When I need to use the handbrake in an emergency, it's not good not to have it working properly."
Capito stood firm on VW's timetable for the upgrades.
"We have a rigorous and disciplined evaluation process and we don't put any parts in the cars until they are 100 per cent proven – and this is not yet the case," he said.
"If we were to put the new system in before it was ready and it failed, that would be a much bigger disaster."
VW technical manager Francois-Xavier Demaison agreed drivers could not rush technical developments.
"We don't know yet when it will be completely signed off or when we will have it on the car – hopefully for the next round in Greece," he said.
"But it's not Seb Ogier or anybody else giving pressure who will decide that date. We have a validation process which we will stick to.
"Somebody has to take the decision and I took the decision not to have it [for Argentina]."
HANDBRAKE CHANGES A TOUGH CALL FOR VW
The new part will be the third evolution of handbrake for Volkswagen. The car started life with a mechanical system similar to the one used on the Ford Fiesta RS WRC.
Having come from Citroen's quicker hydraulic system, Ogier wanted improvements – hence the pneumatic system.
This failed in Portugal when the valve stuck open do to excess pressure (leaving Ogier's car briefly in front-wheel drive) and then it failed because of a lack of pressure – due to a split pipe – in Argentina.
Ogier slid off the road and out of the lead while experiencing this problem.
Demaison admitted he was reluctant to run the hydraulic system.
"It's heavier, it's more complicated and it takes power from the engine to run an electrical motor," he said.
- Sebastien Ogier