rtl simulation and gls simulation

i have write a code using verilog and associate with PLL (using megawizard). when i run using RTLsimulation, it produce correct result. but when i run gls simulation, the output of PLL become high impedance (X), which cause all my output become 0.

i'm using the same testbench for both RTL and GLS.

what actually happen? why PLL can't produce correct output?