Who told you they would last long? M401 is the length of a mini but the diameter of a super-mini. Super-mini carts have extremely tiny juice cups. BTW you can just spend 75 cents for the precut 510 blue foam at strictlyejuice.com and cut those down for M401 carts (1/10 the price of buying the bulk Marineland foam)
Also search Youtube for M401 Blue Foam
I cut mine about 2/3 the inner length of the juice cup, a tiny bit wider and I taper the inner end a bit. I have blue foam in my M401 cart (I am still using my first cart). M401 manual was my first ecig and it got me through the first month but by the end of the month I had bought both 510 and Vapor4Life to try and now I have full setups of those and only vape the M401 at home as an extra flavor I can pick up. M401 has excellent flavor which you may not realize if you have only tried prefilled carts. But after a month, I was pretty tired of messing with the cart all the time! The blue foam helps keep the flavor nice and is easy to take out, rinse and pat dry, put back in. For a full fill, remove the blue foam, fill juice cup 1/2 way with juice, insert the tapered end of the blue foam and push it down into the juice (I use an unbent small smooth paperclip). Then using the paperclip reposition the foam even with the top of the juice cup. Then use the cart-plug gently to push the juice cup back down into the mouthpiece without dislodging the stuffing. Sometimes the blue foam slips in. You could try a full-length plug to start.
Re carts - if your inner juice cups have a flat side and a big 1/8 inch spike on the bottom, those are the old carts. The new ones are no larger but I prefer them for filling - they have a rounder juice cup with no spike. MyPVShop.com's blank 401 carts are the newer type.