TDC GP22 capacitor charging issue

[maoterk]

I am trying to develop a flow meter using the TDC GP22 chip. Does anyone have any experience with it?
The circuit for testing is the same as in the picture attached.
I have succeeded in correcting the frequency of the 4MHz oscillator which means that the communication with the chip and the stm32 i am using is working properly. I can also read from register 1 confirming communication.
When i start the ToF procedure however, the capacitor that is connected to the receiver transducer never charges to 1/3Vcc (checked with oscilloscope) so that the received signal can be then processed by the chip for measurements. It should look like in the second picture attached but the received signal in my case stays around GND. Has anybody had any similar issues with the receiver part of the application?

    thanks