I was using primed Long Rifle cases instead of blanks.

The primed cases are Russian. I just pushed the ball onto the end of the Long Rifle case, smeared a little grease on it and loaded them in the Bearcat Shopkeeper and fired them. Velocity was about a like a spring powered BB gun ... 300 fps or so. 1 grain of Bullseye makes 'em sing!

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Ele era velho.
Ele era corajoso.
Ele era feio.

What pellets are you using? A lot of pellets are dead soft and don't take heavy blasts of air/gas very well. I think you might get better results from some of the heavier pellets designed for the "magnum" pcp rifles. JTS makes pellets up to 25 grains in 22 caliber, if I recall correctly. Or, as Jim suggests, the Gamo round ball ammo as there's no skirt to blow off. Take a look at Pyramyd Air for an idea on the various kinds of ammo available for airguns today. They even offer slugs as well as the traditional skirted pellets.

You have to load the pellets nose to aft and skirt to fore. Folk have been shooting primer-only .22 centerfires thusly for at least four decades. Won't work with the nose forward as you have discovered.

If there is the problem of the undersized pellet not staying back against the blank in your application, then a minute smear of grease/petroleum jelly is usually enough to hold it in place. The disadvantage if firing into the wind or dead calm is the diesel smoke smell from that grease.

B.T.W. the most fun I have had with this theme (blanks/pellets) is with the break-open beretta 21-A. Also works in the taurus .25 acp near copy with shotshell primers. The .25 airgun pellets can be pricey per shot but the big stores (cabela's, etc.) often clearance them for $2 to $4 a tin.