I have tried using my Nikon D5200 in aperture mode with a standard 50mm F1.8 lens at F16 with 2 filters, a Hoya circular Polariser + a ND4. I just can't get the shutter speed slow enough in fairly bright sunshine. 1/8 & 1/4 best. I use ISO 100 though the Nikon changes this to ISO 200. How can I get an exposure of several seconds? Any help gratefully received. Thanks. Roger.
You really only have a few options. Basically, it's all about shutter speed and reduced light, so you can:
Get a stronger ND filter. The "Big Stopper" by Lee is such an option, but even welders glass can do it. You can also stack filters, but I would tend to a single filter first.
Stop the lens down even more which will increase softness, but since you're actually looking for that in a way, it's less an issue.
Shoot in less bright conditions.
Bracket the scene with a sequence of faster shots and then stack them to simulate the long exposure.