How does frequency affect the ability to traverse objects in RF transmissions?
The rule of thumb a lot of people use is that lower frequencies will have better "penetration" than higher frequencies. That's true in some cases, but not all. This is probably derived from calculating skin depth of materials. The skin depth is just how deeply into a material an electromagnetic wave of a particular frequency can penetrate. The equation used when the material is a good conductor is:

$$
\delta = \sqrt{\frac{2\rho}{\omega\mu}}\
$$

where ρ is the resistivity and μ is the permeability of the material. What you should notice though, is that as frequency (\$\omega\$) gets bigger, the skin depth gets shallower. Here's a practical example of what that means: your microwave shoots out radio waves at 2.4 GHz. If you put a giant thick steak in there, and we measure it's resistivity and permeability, we can calculate the maximum thickness of steak you can cook in your microwave. Anything deeper than the skin depth won't get cooked, because all of the energy of the microwave will have been absorbed already. 

There are charts like you mentioned about how well different materials absorb radio waves, but they're not linear or predictable, so there isn't really a rule of thumb that's easy to apply. Here's how well every element in the period table absorbs photons (electromagnetic radiation). The energy on the Y axis is proportional to the frequency:



But this chart of Iron's absorption (according to different mechanisms) shows how things get messier when you zoom in:



But in your application, there's another factor at play, which probably has a bigger effect. When your transmitter starts going in your big facility, it sends off an electromagnetic wave in all directions (assuming you're not using a directional antenna). Those waves will travel through the air until they encounter another medium, like the metal in the containers. When the wave hits that container, some of the energy is absorbed into the container, and some is reflected off the container. The part that's reflected will travel until it hits something else and then some will be absorbed and some will get reflected again. This is called multipath. Your receiving antenna might get a bunch of copies of the originally transmitted signal, all slightly time delayed. Here's an example of what it looks like when an analog TV is suffering from multipath problems:



Because multipath effects can cause waves to destructively interfere with each other, that's probably why you're getting conflicting results. The position of the antenna and transmitter and containers will change the performance a lot, and if things are moving around in the facility, you may get a great signal one moment and then all of a sudden it will be terrible. 

Dealing with multipath is hard, but here are a couple things you can try. Make the receive antenna directional, so it will hopefully have a low sensitivity to reflected signals. If you can get the antennas high up above the containers, that may help too. I would experiment with a 433 MHz transmitter (there are a bunch of companies that make modules) because I think you'll get better performance versus 2.4 GHz or 5.8 GHz.