Assuming it is I2C interface you are working with -
1) You have confirmed pullups somehwere in range of 4 - 6K ohms
2) Confirmed with a scope signals looking roughly correct on clk and data lines,
no excessivley long rise times
3) I2C pins configured correctly
4) Correct address of slave
5) Interrupts enabled if using interrupt driven ISR code
5) Rate settings, hence clk rate, oversampling rate, correct for both
Tedious but you could use an example project on host, then on slave side,
connected to a known device, to see if code is correct on each side.
Just some thoughts, Dana.