On one of my trees the main trunk has stopped growing the past few months. In fact, a few of the suckers have surpassed it now in growth. Any thoughts on whether or not there's a problem? Will the main trunk ever grow? It's a pretty young tree.

Cut the suckers. & divert energy to nain trunk. Would be my choice. nake curting or do air Layers

I airlayered the suckers on my young Longue d'Aout.  But if the suckers are more vigorous, and that is what you want, then you could airlayer main trunk.

You could also let them grow in the bush form as well.   Personally, after 3-4 trunks I airlayer them off.

All good ideas guys, but I'm just concerned that the main trunk is dead or won't grow..

Are you sure the main trunk is dead?  This is a problem.  Be it me, I'd sever at least all of the suckers, and leave just one to compete with main trunk.  This is a big problem.
Suzi

It doesn't look dead.. It just feels like a dud. I've heard of some shoots having this problem.

Several of mine have done the same thing.  I figured they're just lateral limbs that sprout from nodes under the surface.  I don't know why the main stops growing.  They make good air layers with a gallon pot or whatever size is appropriate at ground level.

My plan is what Don said.  Air-Layer the slower growth.  I had a lot of cold damage to my trees this past winter resulting in a lot of die back.  Many of the trees that did not die back, had better growth from the suckers than the remaining trunk.  I've been wondering if they were cold damaged, but just enough to cause some plumbing damage and not enough to kill them off.  

There's no difference between a sucker and a branch and the main trunk.  A sucker is just a branch that started from wood under the soil level.  You could select one of the suckers to be the main trunk.  You could air layer some, all or none.  Shape the tree as you see fit.  As long as something's growing don't worry, be happy.