Receiver Too Weak?

Your receiver is low end and does not come with pre-outs for the fronts/center so you can run them to a powerful external amplifier.

What is your budget?

Get a processor with pre-outs (price can go up quickly).

Here is some affordable stuff

https://emotiva.com/products/basx-mr1l-9-2-channel-dolby-atmos%C2%AE-dts-x%E2%84%A2-cinema-receiver

Or

https://summithifiusa.com/collections/tonewinner/products/at-300-16-channel-pre-pro-releasing-soon

Pair such a processor with one of these amps.

https://emotiva.com/products/xpa-5-gen3

https://www.monoprice.com/product?p_id=15593

https://summithifiusa.com/collections/tonewinner/products/ad-7300pa-amplifier

If you have a slightly bigger budget...

https://www.crutchfield.com/p_642AV10/Marantz-AV-10.html

https://www.crutchfield.com/p_440B15852B/Rotel-RMB-1585-MKII-Black.html?tp=180

etc

If you have an unlimited budget...look at Trinnov, Storm Audio, etc.

Home theater specifically. My biggest complain is that either the dialogue is too low relative to the other sounds or vice versa. I can’t ever seem to get them both to be the proper relative volume.

@easytarget  This is caused by the huge dynamic range of many multi-channel audio track mixes. It’s optimized for folks who want "lifelike" whiz-bang effects at loud SPL. It’s NOT great if you’re trying to get strong, audible dialog without popping your eardrums when Godzilla shows up. It also forces you to push the volume up WAY higher than what you would expect, just to hear clear dialog.

Most processors/ receivers will have a setting for precisely this reason - to compensate for the benefit of dialog. Sometimes it’s called "dynamic range compression" (normally that has bad connotations, i.e. "CD’s loudness wars", but it can be very good here!). Sometimes it’s more wordy-mumbly like "make loud sounds less loud". You need to find that setting, and turn it on! In all likelihood, your speakers and amp are fine and will sound REALLY good once you rectify this. Sometimes just a simple 2ch downmix helps, but you should really look for this setting.

Personally, I hate it when a mix has TOO MUCH dynamic range, for exactly this reason. Most 2ch music formats have a much more reasonable mix for the home environment.

You can boost the center channel (where dialogue lives) to make it more easily heard. Run it into a ribbon tweeter to make it easier to understand.

A simple compander adjusts the dynamic range to your liking.

EasyTarget, most receivers have the ability to control the volume of each channel.  I suggest bumping up the center volume to help you hear dialog, but keep the front R L channel volume the same, so the loud parts of the show don't get too loud.  I find the really loud parts are mostly the R&L channels.  

As to lackluster sound, as GHD said, your receiver does not have power (current) to dynamically drive your R&L channels to their full potential.  And the overall quality of the processing and output of the receiver will limit performance.  

Not all of the issue is the AVR, some of it is the sound engineering on the movie. I run into this a lot. Some movies have a perfect balance between speech and sound, others do not. 
I have a Denon that is supposed to be their "best" AVR for sound (it sounds not-great, but not terrible with music). It has a buttons on the remote to change between movie, game, etc, and when you press a button it brings up a menu so you can choose between different type of Dolby and other interpreters. I just try to use those to get a sound that works for each film, and if that fails there is another button that lets me boost the center.

The last thing is what others have been saying, your amp may not be up to the task of driving your speakers. In my case, my AVR has a line out that I run to my preamp and direct to my monoblocks. The limiting factor of an AVR's output is the power supply, if you look closely at the power ratings of your AVR, you will see it goes down the more speakers you add, de-rate their claimed output by 30% to 50% and you will be in the right ball park. I would suggest no less than 150 watts per channel with all 5 (or 7) channels being driven for those speakers if you want good "life" in your movies.