RGMII output signal of triple speed ethernet can't be captured by signalTap tool

The RGMII output signals ( rgmiiout(3 downto 0), tx_control ) of triple speed ethernet can't be captured by signalTap tool while the RGMII input signals(rgmiiin(3 downto 0), rx_control) are captured correctly. However, in this case, there is indeed some packets come out of netport. Because delayed mode is enabled at the external PHY devices, the rgmiiout and control signal is edge-aligned with tx_clk signal. I'm comfused why the RGMII output signals can't be acquired?