Whether or not you can write data to a Modbus slave depends on the data type you are using.

Only coils and holding registers are permitted to write to. What registers are you trying to write data to?

Is the slave returning any exception responses, which will show as an error on the Modbus master?

Hello,

agree with Smiffy. Just one another idea, it can be simply problem with no correct code on your CPU prepared for different tasks - read/write. With this procedure is clear that you need have all tasks closed with read procedure to start write task, as example. To my opinion problem can be here or with that Smiffy's mentioned things. Simply your code need to be prepared due expected data type with that frequency so here is expected generally DWORD as base data type or listed in documentation of device, of course as Holding Register access.

...I also checked the wiring connection and it is correct.

this is not important generally here because if you have it incorrectly wired, you can't also read data