we are developing pedometer as wearable.
It is hard to say where the problem might be without knowing more details. A schematic as well as screnshots illustrating what is going on the bus would be useful. You can email me directly or create a standard ticket if you would like to keep them confidential.
A couple of general hints. The MMA9555L is operating at a supply voltage of 1.8V +/-5%. This is also the nominal voltage for all digital I/O pins so please align your board supply tree accordingly.
Which slave interface mode are you using: I2C or SPI? Keep in mind that the level of pin8 (RGPIO3) during reset determines which slave interface mode will be used by the device (set pin to high level for I2C mode).
No specific settings for the host MCU, MMA9555L slave 7-bits I2C address is 0x4C (which gives 0x98/0x99 as I2C "8-bits address byte" content).
I am attaching a PDF document that gives some insights about communication protocol with Mailbox.
Again, more information such as the schematic and slave interface plot/log are desired to help you further.
PS: If this answer helps to solve your question, please mark it as "Correct". Thank you.