Making everything audible in the mix is not always possible. Elements that share frequencies will mask each other.
The most crucial part of the mix is not actually in the mixing phase itself, but in the composition, instrumentation, and arrangement phase. Experienced composers will give each element its space in the frequency spectrum, so there is little to no masking, and when there's masking they use it to their advantage in the form of textures, ensembles.
The masking effect is the number 1 reason why people end up turning everything louder and louder in the mix. Once one element is audible it masks the other ones that are near it, so you turn up one of those or all of them, and now they are masking something else, and so on.
You have tools to improve this situation, but there's a limit on what they can do depending on the severity of the masking. If the problem is that the elements are too close to each other in both time and frequency, then what you want is some separation.
Panning conflicting elements to opposite sides, but your mono mix won't have this benefit.
You can give some frequency space with EQ (accentuate different bands in different elements, for example), but too much of it and you'll be messing with the timbres.
You can use sidechain compression, but you'll need to choose which element will be quiet while the other is on (works great with kick and bass).
You can use an harmonic exciter to make an element cut through the mix, but those added harmonics might be masking something else.
You can also consider getting rid of some elements, or redesign them in a different frequency range (check which bands are available), or use them in another point of time. If the problem is too big, it might be worth to re-arrange the song.
From SoS Mix Mistakes:
The roots of many a mix problem can be traced back to the musical arrangement, and this simple fact renders many of the budget productions I hear effectively unmixable. If your song's verse has more guitar or percussion layers than its chorus, you're likely to face an uphill struggle if you want the chorus to arrive with a bang. Likewise, there's no sense in having different guitar and keyboard sounds competing in the same pitch register if you want to keep any separation between them in the mix. And unless you create some sense of build‑up in the arrangement itself, it's unlikely that you'll hold the listener's attention all the way to your final chorus.
From SoS 20 Tips on Mixing:
Try not to have too many instruments competing for the same part of the audio spectrum. The mid-range is particularly vulnerable, so try to choose the best sounds at source. You can improve the separation when mixing by using EQ to narrow the spectrum of the sound you're working with. Try rolling off some low end and occasionally taking out any excessive top end. This is sometimes known as spectral mixing, where each sound or instrument is given its own space in the audio spectrum. A good example of this is the acoustic guitar which, in a rock mix, can muddle the low mid. If you roll off the low end, you still get plenty of definition, but the mix will seem far cleaner. Sidechain filters on noise gates (set to Key Listen mode) are often very good tools for trimming the high and low ends of sounds without unduly changing the section you want to keep.
The possible problems and solutions don't end here, but that's the most common cause of your issue.
Masking aside, a technique that will make your life many times easier is the use of buses and/or mix groups. For example, route all the synths to one fader and mix them separately (mix only the synths). Repeat with drums, vocals, maybe you want to group kick and bass and all other low freq elements, etc. Then you mix those groups together.
From SoS Mixing Essentials:
Divide And Conquer
Now that modern DAWs are capable of recording huge numbers of tracks, modern productions seem to want to use them all! You might find your mix initially seems unmanageable, but you can make life much easier by separating key elements of the mix into logical subgroups that can be controlled from a single fader. The obvious example is the drum kit, which may have as many as a dozen mics around it or multiple tracks of supplementary samples, and you clearly don't want to have to move a dozen faders every time you wish to adjust the overall drum kit level. There are two ways to do this in a typical DAW, one of which is to group the faders so that when you move one, the others move proportionally. The other way is to create an audio subgroup and route all your drums via that group, just as you would on a typical analogue studio console.