Unable to push mineral oil from hydraulic brakes calliper bleed screw

[motordriver]

Hi! I’m new here and I have a GRX810 groupset.

I am trying to bleed oil from my rear hydraulic brakes and I got the oil to drain from the calliper hydraulic bleed screw. Now I am trying to push new oil back from the bleed port but I am not able to, the syringe is not moving at all no matter how hard I push. So I have checked the following:

• calliper is not blocked, oil can flow from bleed screw to cable port
• cables are not blocked oil can flow from one end to another
• I've also checked the diaphragm and it does not seem torn or damaged.
  

I am suspecting there is a blockage somewhere within the lever. I am also not able to make oil flow from the funnel into the the system.

Any suggestions or experience to share?