CST dual polarized H-slot antenna is very well matched but has poor total efficiency

As the title says I am doing a project to design an antenna for base stations in mobile communication using CST software. I have attained a very good match at both ports (I have attached a word document with screenshots). The problem I am having is that the total efficiency at the resonant frequency (3.4GHz) is just above 20% when ideally it should be around 70%. I am not sure what the reason for this is I am using FR4 for the substrates and PEC for the ground and patch. There must be some sort of error in the design but I have no idea what it is, I will attach my CST file.
Any help would be greatly appreciated.

Hi,
My CST version is lower than your installed CST, I have not been successful to open your CST file. However, your have used FR-4 substrate for frequencies more than 2 Ghz? its loss tangent decreases the efficiency of the antenna. I would prefer to use Rogers series instead of FR-4.

I have reran your simulation and added monitors for the e and h field at your resonant frequency. This allowes to display the power loss in dielectrics (In 1D results -> Power). And the results confirm the assumption from baby_2 that the dielectric is the main cause for your decreased efficiency. I have attached an image of the simulation result.