I want to know what the issue is because kicks and synths phasing each other out actually sounds like a pretty cool effect for a track.. probably not the one you're making though.
I don't think there's an actual reduction in volume going on, what you describe sounds more like frequencies masking each other. In that case, making anything louder will only make it worse..
Think of it like this: the volume is there, your ears are simply not picking it up, that's why it's called masking. They give priority to the synth as it were.
Look for the 120-450 hz range. The stuff going on here is probably both what makes the initial impact of your kick and the bottom end of your synth sound. You always need to do
some tight EQ'ing this range if you're making bass heavy music. Getting deep subs is easy, 60Hz sine or square tone, filter on top: done. 70-120 hz is generally gonna be your kicks sound. But that range right
above is what gives a lot of sounds their weight and body. Add some vocals and literally every important sound in your track will be fighting for this area.
So to solve it in a simple way, look for the low fundamental frequencies in your synth sound (the biggest bulge or spike on your spectrum analyzer, and try to cut that down. Take note of the frequency you're cutting.
That should already improve things a lot. Next you go to the kick and apply small cuts a little above and below the frequency you just cut on the synth, to give it a bit of extra breathing room.
There's more complex ways to do that are more transparent, but this will do in most cases. Like I said, this is the busiest frequency range of all so it's good practice to keep things tight and controlled from the start here.
PS. if you're looking for that stark, transient rich synth-drum sound... nothing beats getting it from a synth. Samples are a nice facsimile to start off with,
but using a synth gives you much more flexibilty by tweaking parameters, precise tuning, running them through your effects. And way better quality.. everytime you edit a sample you degrade it in some way.
Everytime you edit a synth, it just spits out a new untouched version of it. Do it for an afternoon and you'll have a sample pack worth of kicks.
