Low satellite signal strenght is the most common problem in RTK systems. If you don’t have the minimum signal strenght, it’s simply not possible to calculate centimeter level position.

Antenna and board installation are important to guarantee this quality of signal reception.

There’s a very good application note done by u-blox, about general rules for GPS/GNSS antennas:

https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_%28GPS-X-08014%29.pdf

And a more specific one explaining best practices with GPS/GNSS RTK antennas:

https://www.ardusimple.com/wp-content/uploads/2021/01/AntennasForRTK_WhitePaper__UBX-16010559_.pdf