When printing white only, you lose a layer of toner through the TransferRIP as you aren't putting down any coloured toner. Below is a screenshot of the recommended settings for step 4:
When pressing the A and B paper together for white only designs, change the pressure to 5/6 bar. The white toner is more brittle than the other toners and sometimes needs a firmer press.
Also be sure to pre-warm the B paper beforehand. Follow this guide for pre-warming advice.