well…it could be a number of things if you keep jamming eqs together when you’re in the mix. if i were to venture a guess, you’re either trying to force the mix at a time when the harmonics are just not copacetic enough to not sound all fubar. if you’re playing rips, they’re probably lossy transcodes or worse, which often leaves dj’s in a tight spot when one track is normalized and compressed properly and the other is too quiet, etc etc.
drum and bass is not like techno or house in the slightest, so you have to pay more attention to the phrasing of the tracks themselves because they’re doing the majority of the transitioning for you. with 4/4, there aren’t nearly as many breakdowns/responses/phrase shifts because you’re the one creating them with loops, eq adjustments, processor effects, etc. sometimes with drum and bass, you’re better off just cutting the cued track in, lows/mids dialed down and gradually increase while you’re dialing down the master track eqs. then when you hit that ambient breakdown with no drums/bass (hypothetically, mind you) you crank the bass/kicks/gain up to compensate for the decrease in volume of the master track. you really have to pick and choose your tracklisting so every track compliments the one before it and the one after…which is why i recommend just taking a crate of 30 or so tracks and mixing them while recording; then, you just listen in earbuds for the discrepancies you missed while monitoring during the mix. i’ll cut a mix and listen to it 10x or more, dissecting what worked and what didn’t w/ an anal-retentive attitude about how the tracks fit together as a “theme/vibe”, the “emotion” the tracklisting invokes w/ the mixing style and order of tracks and how i eq.
in the end, you really just have to know your tracks more than anything. if you’re wanting to put out a demo or play out live, it’s important to have a “concept” of what your 1 hr set (or more) is going to sound like, how it’s going to grab the attention of your audience and also think if it’s something you’d like to hear any other dj play when you’re in the audience. with so much diversity within the genre, especially with the formulaic structure of how drum and bass is generally produced, you’ll start to notice patterns: when drums change up, when kicks are muted, when the rides and hi-hat loops are inserted at a specific measure to push the track forward and break the monotony of the earlier measures.
generally, there’s a lot going on specifically due to the tempo, so you have to mix fast and clean if you’re not familiar with the subtleties each track has in relation to the next. set up cue points, use basic addition/subtraction to figure out when/where to bring in your mix then set cue points for when you transition the bass lines from the master to the cue…rinse/repeat 
build your crate so that four/five tracks mixed together all sound like one really long track then segue into a lower/higher key (i think they call it “hotmixing”: take a 9a down to a 2a/8b to a 3b; by dropping/adding semitones but staying within the major/minor key you can dial down/up the vibe. with key codes, this is generally done in increments of 7 and 5 key codes along the wheel), then more up/down the wheel for another 4/5 tracks then hot-mix again. by doing that you create peaks and valleys, generating an emotive response from the listener more often than not. that diversity and attention to detail goes a loooooooooooooooong way with drum and bass imo.
i hope that makes sense…all things considered, i’ve been up for like 36 hrs straight trying to catch up on homework before the semester ends, so bear with me if my advice is a little scatterbrained 
i think the best thing to do is keep in mind what i posted here, then really give an in-depth listen to that mix i have up on the zaptown site, then check out the others i have up on soundcloud. listen to them from beginning to end and the method to the madness will start to make more sense. then start your own practice session with my tips and the mixes in mind, record, and listen/digest it several times. then you can modify accordingly to seperate what worked, what didn’t so on and so forth
if you still can’t figure it out, maybe try telling me what tracks you’re having the most problems mixing, link me to them and i’ll see exactly what it is that is giving you fits 
that way i can try to put them together and show you how i would cue/eq/fade/cut/etc…i think that makes the most sense if you still can’t follow how i’m describing my thought processes.
ez,
-g-
ps: i also recommend downloading podcasts from artists/labels you are digging and giving them an in-depth listen, too. pay attention to how the pros are formulating their mixes and implement their technique into your own. after wrapping your head around the sound for awhile, it will all start to come together. promise!
