SPI problems

I’m having a small problem with receiving SPI. this is the comms between two AVRs, identical settings for the SPI bus with the exception that one is set as a slave and one as a master. I send four bytes, 90% of the time it works perfectly and the timing looks like this, the “LED1″ pin I bring high when the SPI Receive interrupt is triggered and I clear it at the end of the interrupt. -

SPI Good

10% of the time it ‘misses’ receiving all four bytes, looks like somehow the SPI receive interupt isn’t always being triggered.

SPI Bad

Anyone got any idea where to start looking? I’m perplexed.

 

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