Interrupt level 5 a step too far?

And C programming was going so well! Was running SPI1 interrupt set up as priority 3,1 and an external interrupt set up as priority 4,1. SPI wasn't getting serviced from the external interrupt routine, so moved SPI up to priority 5,1. Now I cannot get code into the board. Have I moved the priority level too high and are now interferring with USB coms and the bootstrap loader?

The boot loader does not use interrupts, its kept very simple so that it will fit into the protected boot space. I think that the not being able to get code onto the board is unrelated and probably due to something else - the boot loader is entirely separate from the user code.