Have you ever topped them? Ours are much older, but we have to top them and they are quite bushy now.

Yes, I agree, trim them at the top and even on the sides a little and it will encourage growth.

I think that you planted them too close. Looks good in the short run but not so good in the long run. They will be competing with each other and I'm thinking that none will be as healthy and strong as they should be. IMHO

I've been wrong before and could be wrong now so take my opinion for what it is worth :-)

And it appears our too close to that fence. Then should have been at the very least 18 in off that fence. If not, three foot to be able to grow bushy. They don't like to grow into things. Podocarpus calls for a minimum of 3 ft apart

Make sure that the top of the plant is pruned so that it’s narrower than the bottom. When the top is too wide and/or too dense it blocks light from reaching the rest of the plant. At this point I’d drastically prune them. I’d follow the tallest/bushiest branches to the lower part of the plant and cut them so that they’re maybe 6” to no more than a foot or so above the ground, varying the height so that they’ll fill in in a way that looks more natural. I’d prune the rest of the taller shoots to a point below where they’re most bushy. If you use an electric hedge clipper such as are typically used by your average landscaping crew you’ll end up with what you now have.