1. Are you using a quality motherboard? Using a cheap Z board might lead to worse or unstable overclocks. You want quality power delivery and a beefy VRMs.
2. Are you manually overclocking the chip? Using something like AI Tuner or the "easy one click OC" are not going to give you good results. You're going to need to do specific research on your motherboard to really fine tune the OC in the bios directly.
3. It really is a lottery, some chips can do 5.0 on all cores at 1.4 or less, some can't. But there is another option rather than getting a better motherboard or better chip. Try just overclocking 1 or 2 of the cores at 5.0 and leaving the other ones at 4.8 or lower. The benefit of 5.0 on all cores only works for programs and games that utilize them all, whereas windows and most programs (and some games) really only use 2 cores. Having the 5.0 on those specific cores will give those programs the benefit without having to have all the cores at 5.
Hope any of this helps.