You may need to back out the schrader core a little or put a VCR tool on it and loosen the core while you're working on it.
Personally I don't like low loss fittings. I prefer ball valves on the end of my hoses. Though, that probably won't solve your problem.

 

I have occasionally had that happen when using LL fittings. I use ball valves now and can screw the depressors out a bit if needed.

 

I like to see how far I can throw low loss fittings into the weeds
Ball valves rule


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Mini's are pretty tight to work on. I always pull the core when I evac but I am pressure testing at the end of the day. When I pull the core I have to take screws out to gain access to the port. Pain in the arse. I have had problems with the ball valves. I have the YJ and the handles tend to leak. I also have one that when I tighten on the port it blocks the flow.

 

The YJ hoses, etc. have adjustable core depressors, they just screw into the rubber seal. Just need needle nose pliers to grab the little bugger.

 

Low loss fittings fail regularly, get some short hoses with ball valves.

 

I prefer ball valve hoses as well, but often have the same problem of not being able to depress the valve core. The worst is when the connection partially works, making me think a pressure is much different than it actually is.

The problem is often a bad core, I just changed one in, ironically, my valve core removal tool's side port where the stem was so short that depressors couldn't make contact. It was not that the original core was screwed in too tight, it's stem was visibly much shorter than on the replacement core. I think the stem needs to be nearly flush with the port's opening to work properly.

The other thing I've run into is narrower than standard openings in a port that interfere with relatively wide depressors. If the port is cored, this interferes with being able to depress the core. Whether or not there is a core, (say a valved port like a king valve) the depressor blocks my hose connection on these narrow ports before the hose gasket can properly seal! Why does this happen?!?! I think in some cases a gorilla has come before me and tightened a metal cap on so tight it has deformed the port opening. In others, it is just the way the port was manufactured. These connections will sometimes hold for a pressure test, but not a vacuum, FRUSTRATING! Once I figure out what is going on I reverse my hose so as to use the end without a depressor, but that doesn't work if I'm just putting on a stubby gauge with depressor for a quick system check, or there is a core...

 

I have been using the "JB" brand hoses for the last 5yrs and I love them. Low loss type without ball valves. 98% a perfect connection to tap. I use mine almost daily. I used Richie for years before switching to the jb brand. I'm not brand loyal I like the best tool for the job. These hoses will still swivel while connected and are rebuildable when needed. Using the Richie I replaced hoses every 6 months, under warranty even a few times. 5 years using the "JB" I have replaced once. And rebuilt the set I have now just 6 months ago. I really like these hoses. Usually they do cost a few dollars more than the low loss yellow jacket hoses but ya get what ya pay for. Here are the ones I use.

My secret weapon is HVAC Talk.

 

I agree with all the others who recommend the hoses with ball valves over the low loss ones. I have yellow jacket brand and have never had a problem with them not depressing the schrader.

 

I like the black hoses with the ball valves from United.

 

I don't buy hoses with valves or low loss attached. This is normally where your failure is so I buy them separately. Ball valves always leak at the handle and sometimes reduce flow.