First post, so hi thereeee (:

Anyway, lately I've been wanting to write more technical music, but my problem is that all my songs end up sounding the same, which I don't want.

So my question is, what can I do to prevent this happening ? I've studied a lot of theory, but I still can't quite get the diversity I want.

If it is of any help, I'm a metal-based guitarist, and my main influences are The Faceless and Whitechapel. (Yes, I know Whitechapel aren't Tech Death).
I can't help you with writing metal specifically, but I can offer you a little general advice: if there's a certain way you go about writing songs, flip it on its head and try different approaches. Do you come up with a chord progression then create melody then lyrics? Try writing a melody first and harmonizing under it. Start with a drum beat? Then try writing a different instrument first. If you want different results, you gotta try different methods.

Analyze your songs, too. Why do all your songs sound the same? Similar chord progressions? Always using the V - I (or i) cadence? Try a IV - I or VII - i. Move your chords by thirds. Melodies getting stale? Try using more skips (intervals or a third or greater).

Whatever you're doing, do the opposite.
Nothing that is worthwhile in life will ever come easy.
like they say with theory, learn the rules so that you can break them. when writing try to not think about theory and just write. theory isn't really a set of rules, it's just helpful advice. so just play what you feel coming to you man.
If you keep sounding the same, then next time you do it, stop and take it another direction.

"Oh, I'm going to the b6 again... "

... don't.

More general suggestions -

Write using new scales.
New tempos.
New chords.
New time signatures.

... and keep going!
Personally, I despise Technical Death Metal but I may have a few pointers:

Tempos and time signatures are a big factor if you want to write some challenging phrases. Death Metal tempos are typically aggressively fast but try playing in half time speeds too and vary the tempo several times within a song via taste and flow. Experiment with eight time signatures like 7/8 or 9/8 and, you can change the time signatures any measure as accordingly. Break away from 4/4 and find something new.

One thing, if you really want to get creative and allow theory to work for you, trying studying counterpoint and harmony to come up with not only more challenging pieces between yourself and another player but to spice the music up into something interesting! I don't really know if many Tech Death bands employ counterpoint but you should give it a try.

Most Tech Death riffs honestly sound like advanced guitar exercises to me...
We're all alright!
Try learning some new techniques and then incorporate those into your playing. Try stuff like string skipping, more unorthadox sweep patterns, new scales, etc.
Write music and not guitar exercises. All the great tech death bands like Demilich, Gorguts, Spawn of Possession, Necrophobic, VUVR and Capharnaum are good because they wrote quality music that's difficult to play, instead of technical stuff for the sake of being technical. That's exactly why modern technical death metal is considered such a joke. Once you figure out what you want to write, if you feel it needs to be technical then do that. You can't force it.