The RN4081 is notorious for a hard draw. Many report having to take multiple primer puffs to get that great vapor that the model's atomizer DOES produce. There are a few things you can try.
Most simple is line up the cartridge. The mini models have less of an airflow channel so this can be important. This link is for a 401 atomizer, but it gives you the basics of how airflow works
through the atomizer and cartridge:
http://www.e-cigarette-forum.com/forum/tips-tricks/6288-m401-evo-tip.html
Most likely it isn't the batteries, it is the atomizer so try this next. If the atomizer is clogged that can turn a hard draw to nearly impossible. Have you blown it out? Are you using VG liquid (which will clog an atomizer quickly)? A quick soak in some Pure Grain Alcohol or good quality Vodka can help.
Blow out any liquid that you can, soak for a few hours, blow out PGA liquid well and shake like a thermometer to remove all the liquid you can (it will evaporate) and prime well with PG or your
e-liquid right away (especially if storing the atomizers).
It may take adding a drop or two of your liquid to get the atty to produce vapor well again. Take care not to vape it dry when restarting it.
If you don't have alcohol, you can use very hot water to help clean out liquid. Blow out well after soaking, and shake as directed above. Dry immediately with a hair dryer or put in a warm toaster oven at 200-250 degrees for 15 minutes. Prime well with e-liquid right away when dry.
Another thing to check (and good maintenance for auto batteries) is that liquid may have gotten inside your batteries. Twist up some tissue into a long thin string and insert inside the battery and stand Ash Cap end up over night to help remove any liquid inside the battery.
If after cleaning you aren't happy with the performance, perhaps it is time for you to consider a move away from this model, rather than to keep investing in a product that isn't working well for you? The difference can be like night and day with some models.
By the way, the RN4081 discussion area is here and there are some terrific tips and tricks for this model there should you decide to persevere.
RN 4081 *Super Mini* - E-Cigarette Forum
Happy Vaping!
