Prostate simulation is more about rhythmic
pressure than anything. Simply having an inserted object pressed against the prostate can provide some stimulation and a pleasurable feeling of fullness, but the orgasmic effects of this are limited without a back-and-forth or grinding application of pressure against the prostate. Vibration might be a nice sensation inside the anus, but it contributes next to nothing to the orgasmic contractions that actually "hitting" the prostate causes... It really takes a manual, rhythmic pulsing pressure on the prostate to produce the kind of contractions that lead to orgasm and pre-ejaculation.
TL;DR: The rhythmic motion of "hitting" the prostate is what causes the on-the-verge, orgasmic sensation, spurting pre-ejaculation (or even urination if you're not careful
) and eventual complete orgasm/ejaculation.
Buttplugs aren't the most ideal toys for prostate stimulation. They offer a sense of fullness, but don't really target the prostate the way a dildo or a prostate massager designed specifically for targeting the area are will... If you can angle the buttplug so that the pressure at the tip is directed towards the prostate, or at least in that general area, it might improve its ability to stimulate your prostate. I wish I had a better knowledge of anatomy terms... It'd make explaining this a lot easier. Hopefully what I said makes sense.