Migrate from SPICE MOSFET Level 6 to BSIM4

Could some SPICE guru please help ? I am using ngspice-20. I designed a phase frequency detector(2 D flip- flops and a NAND gate in feedback loop to reset the flip-flops) with SPICE level 6 NMOS and PMOS models, and simulation results were exactly as expected. I then modified the L and W values to sub-micron sizes and used the BSIM4 model from Berkeley. Now, I am not getting the expected output, as with the level 6 design. Is there something that I am missing, and is the only way to resolve this problem is to keep varying the transistor sizes (L and W) ? Any hints, suggestions would be of immense value. Thanks in advance for your help.