Generally, a new magazine is going to be somewhat stiff.... Eventually, that'll go away.
Besides the 10# sledge out in the garage (works well, PITA to carry), the magic is to put the mag into the gun with the slide open, and just close the slide, chambering a round. Don't try to then top off the magazine, and don't try to insert a fully loaded magazine into the gun with the slide closed. (Some people prefer to NOT chamber a round....)
DO top the magazine when it's out of the gun, and in storage.
Eventually, the spring will soften up enough to make things work as advertised.
I don't recommend cutting a coil off the existing spring - eventually, when the magazine spring gets it's groove on, it may be too weak to be reliable.
Confession: I don't have a Shield yet. I do have three M&Ps, and a bunch of single-stack and double-stack 1911's. I also had a Walther PPS. All of these seem to have had issues of this type with new magazines. Just IMHO, this is inevitable with a double-stack magazine, and likely with a single, unless somebody's spec'd a light spring, which may not be right.
DO be patient - it takes a while....