"Chaz
This normally happens if you have built up a good car for the next season. For example Ben, Last season I had a 60% car even before we had finshed the last one. If that happens, I have found that it takes even longer to develop. So, what I suggest is to put most of your designers on 1 part and do it daily as that is what I did.
I'll just tell you if you're confused. Not the season we just had, the one before that.. I already had the 60% cap for the following season (the one I just won) however the parts took longer and only went up by like 2%
However, because I was in the title battle, I didn't focus on the next season car (so for this season coming up) and yesterday, I had a 25% car for this season. BUT, Now, Today I got a 15% increase in parts and I can have like 14 designers on 1 thing and get all my parts at once with higher upgrades.
So from now on, I may attempt to build a poor car for the next season but in the 'break', I can build it up.
Is this the reason why my car is only going up by like 2% at a time at the moment for next year? All parts are 100% (for this year) except 1 (which is 98%)? Really odd, and I really hope thy get bigger gains in the off season before next season.