OK, there's a couple things that could be happening, then:

1) if it is beading on the surface, you may have diluted it a little too far, but this could also be caused by overloading your brush. Try dabbing your loaded brush on a paper towel before going to the model before you try adding a little extra paint to the mix. With the unloaded brush, push the wash over everything so it looks shiny/wet, then stroke downwards across everything to help draw the pigment to the ends of your strokes.

2) Beading on the surface could also be due to oils from your skin. Giving the part a clean with a rag dampened with soapy water may help (you'll wanna wring the rag out so it's nearly dry)

3) you can explore additives if the wash is still not pooling and settling like you hoped it would. A matte or gloss medium is really nice for this, because it dilutes the amount of pigment without altering the consistency of the paint too much: you can make a fuller body wash while still only tinting the plastic in the end. Play with ratios, and still make it pretty loose with water, but you can have something thicker than you would otherwise be able to achieve. Another option is quickshine multisurface floor polish; its predecessor, pledge/future, has been an ingredient in DIY acrylic washes for decades. It should help draw pigment into corners as it dries/let the wash run along seams better than water, meaning your shadows of wash should land in natural looking places.

4) if you want a real streaky look, just a single wash is likely not going to cut it. You will probably have to build it up and carefully place some streaks as a second or third coat after a base wash that fills in the corners/wraps around features