I've designed a number of boards using, mostly, AVR 328Ps. All of them have worked flawlessly, except.... I've never been able to burn bootloaders into them using a USBTinyISP. I have been able to use the same USBTinyISP to burn bootloaders in several Nanos and ProMinis, though I have also had ProMinis and Nanos that would not allow me to change the bootloaders.
What is the trick to making this work? I have a 6-pin connector, wired up exactly as it is on the Nano. I have nothing unusual connected to the 328P that should interfere. The RST pin has a resistor and cap, exactly as on the Nano. Yet, it does not work. I MUST be missing something...