External interrupt problem of px4flow module

Dear all,
I have a question in terms of the external interrupt (EXTI in STM32F407) handling of px4flow.
Let me depict the details:
I want to add an additional interrupt in the source besides USB OTG and DCMI DMA and I2C1. The interrupt is an external interrupt (EXTI15_10_IRQn) with pin connected to a target SPI sensor. However, we observe a confusing problem: The external interrupt we just added can be triggered only when the px4flow module is connected to a PC via USB, and everything works fine. But, it can not be triggered once the usb dongle is unplugged from PC, and the EXTI15_10 interrupt service routine cannot be entered anymore. It is only workable when the USB port has some devices connected.
I speculate that it might be the USB OTG interrupt has higher priority than my customed EXTI interrupt, so I changed the specific configuration in usb_bsp.c, making its NVIC priority lower but still cannot get it to work.
I hope someone could help me with this. I know it is a bit confusing, and if I do not make this problem clear, any suggestion is appreciated. Thanks!