Well when I first started out, I couldn't afford to do more than a couple pills a night as they were 20 bucks a pop at the time in LA, so for a while I was only popping 1 and following up with the other a couple hours later.
Eventually when the prices dropped and I started buying in bulk, I'd go nights taking a whole 10-pack throughout a 16 hour period... but that's not to say they weren't good. I'd blow up off 2, but when you start coming down and have lots left over, the tendency is to re-up.
After a lot of years like that, still if I come across good pills (that is, pills with at least a majority of the content being MDMA or MDA, most verified via test kits) I still only need 2 to get where I like to be. (Which is a pretty intense roll) But it depends on how much time I give my tolerance to settle down. (A couple months at least.) Now days it isn't an issue, because I rarely do it anymore.
If you're letting your tolerance settle and you're still not getting off with your normal dose, barring some issue like interference from certain medications, (and keeping in mind the contents of your stomach... yes, empty is better) then the most likely thing to assume is that your product is not what you believe your product to be, or at the very least, it's not the quality/quantity you believe it to be.
What kind of dosage do you normally take to get off? And how much time are you giving your tolerance to settle?
Also, how sure are you of your product?
P.S.
I have never done or seen 'liquid' MDMA. I know pure MDMA is in crystaline form, but I've never done pure MDMA crystals/powder either. Most pure I've come across was unstamped wafer-like pills that contained nothing but MDMA and filler.
Either way, as long as MDMA is in whatever medium it's in, then you're taking MDMA in whatever ratio-to-filler the presser has decided to use. The problem arises when other substances are substituted (BZP, TFMPP, caffeine, dxm, etc) at a higher ratio to MDMA/MDA or even with no MDMA/MDA at all.
You can purchase test kits from the internet and test quickly and easily for specific substances in your pill/substance, but the only way to get the ratios and broad-spectrum substance analysis is by sending it to a lab.
EcstasyData.org: Ecstasy Lab Testing & Analysis Results - Ecstasy Pill Reports is a good resource for broad-spectrum analysis and ratio results published for known pills that have been sent in.
Pill Reports - Ecstasy Test Results Database by Enlighten is another good source for identifying pills in your area in a certain time-frame that have been reported on by users of the pills. Some of them even test the pills and provide pictures. Great resource if you want to know if a potential product is worth buying. 
And she only had 3.
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