I seeded LNCaP cells in transwell (medium: upper 1% FBS; lower 10% FBS). I found only few cells in the lower side of transwell after 48 hours. I performed the test with other cell lines obtaining good results.
I've had great success performing transwell assays with LNCaP cells using the Boyden Chamber method. I typically plate 6-8x10^5 cells in a 24 well transwell insert using an 8um pore and allow them to migrate 24 hours. Higher passage LNCaP cells (Passage 80ish) will migrate much faster on average than lower passage (passage 30ish). I've carried out hundreds of transwell assays with LNCaP cells, so if you're still having trouble feel free to contact me.