Vortex Sound Research

Hi, my favorite type of patches are the ones with evolving filter sweeps but I can't seem to program them. I've somewhat learned to do it on my Virus TI2 but now I'm trying to do it on my Prophet rev2 and can't seem to get the sound I'm looking for. I currently have a basic arp going and I want to program the cut off filter to slowly open and close repeatedly over time. I thought I could just assign an lfo to the cutoff and be done but it's not giving me the effect I want. Any tips? Thanks.