Then you got what you paid for.
You have a 3-Star Nissan Skyline BNR34 GTR.
It takes 15 blueprints to stage up to 4-Stars. It takes 25 blueprints to stage up to 5-stars.
You bought 40 blueprints for 2600 Gold. Now, your Nissan Skyline BNR34 GTR has 5 stars.
You need to buy another 40 blueprints to get 6-stars.
Here's the breakdown of the required blueprints for the Nissan Skyline BNR34 GTR:
Stage 3 = 15 blueprints required
Stage 4 = 15 blueprints required
Stage 5 = 25 blueprints required
Stage 6 = 40 blueprints required
For completing Chapters 1 - 4 (no blueprints awarded for completing Chapter 5), you'll get blueprints:
Chapter 1 completed = 10 blueprints awarded
Chapter 2 completed = 15 blueprints awarded
Chapter 3 completed = 15 blueprints awarded
Chapter 4 completed = 15 blueprints awarded
Grand Total of 55 blueprints for completing Chapters 1 - 4 (which covers Stages 3 to 5).
You'll have to spend 2600 Gold on buying 40 blueprints if you wanted your Nissan Skyline BNR34 GTR to have 6 stars.
Now this is where you make the mistake. You bought 40 blueprints when your car wasn't even 5 stars prior to buying the blueprints. That is why your car had 5 stars instead of 6 stars.