By the sounds of it, you might be trying to fit the wrong mag into your Sht.LE.
The problem might be that it is a mag for a No.4 rifle. No.4 mags tend to be wider and more squared off at the front of the mag body. It does vary slightly between manufacturers. Sometime you get lucky and a No.4 mag will fit in a No.1 rifle, sometimes not.
There are a lot of No.4 mags floating around on the loose that were factory converted from No.1 as a war time expedient. The lower half of the back rib was removed, ground flush and the mag refinished.
If the mag has been converted (to No.4 style from No.1) the back rib can't always be used to identify the mag as it might be ground off and half of it gone. This can create confusion in that some people believe that their No.4 mag fits both rifle models, when in fact it is not an original No.4 mag, so they are puzzled when another what appears to be identical No.4 mag doesn't fit.
So we can positively identify the mag by looking at the auxiliary spring attachment tab at the front.
At the front top edge, there is a little auxiliary spring that clips over the front edge (or there should be) and it sits inside the front face of the mag. The bullet tips will bear against it and the bullets are prevented from hammering the front of the soft metal of the mag body under recoil. It is retained by clipping over the mag body edge and snapping onto a little 'pimple'.
Without this auxiliary spring, this hammering by the bullets can peen the front of the mag outwards increasing its size causing a problem as you describe.
Auxiliary springs on Sht.LE mags clip over a tiny round boss pressed out of the metal and corresponding round hole in the aux spring tab, No.4 mags have an oval boss and slotted hole.
If it is a No.4 mag, that might be the source of your problem, the nose is too wide. If a No.1 (Sht.LE) mag, then something has become distorted. Then the fix is to get the hammer and hardwood drift and 'encourage' it back into shape.
Armourers would have a hardened steel mandrel over which they would slip the mag and pound on the thing with a hammer to get it back into spec.