Let's assume your terrorist group has a viable suitcase nuke. Never mind how you know it will work, just assume it will.
Every other terrorist cell in the world is going to want your nuke and will do anything to get it. Your nuke is going to be very much in demand, and you are going to have to protect it until you can set it off. If any of your buds even think you have it, they will be after it, and you won't be aware that they know you have it. Every other cell will imagine that their use for your bomb is better than yours.
You will be under tremendous pressure to pop it off as soon as you can, or someone else will get the glory.
So if there are viable suitcase nukes, anyone getting one will have a short time period in which to use it, probably months. You certainly won't leave it in a U-Stor-It for a year or two waiting for the best time.
Now let's consider how you will know it will work- do you think a Soviet suitcase nuke has a "test" button? You have the conventional explosive, which can degrade, the trigger switches for the conventional, which degrade, the battery (you think they run on a pair of 9's or maybe 4 AAA cells? You gonna replace a Soviet nuke military battery at Radio Shack?) and the neutron generator, which depends on a short half-life isotope. Electrolytic caps degrade slowly, and the radiation from the pit degrades everything including the electronics. Our nuke arsenal is constantly being repaired and maintained. It is very unlikely that a small portable demolition bomb will be operational after a dozen years of sitting around.
So maybe terrorists have already tried one or two- taken the elevator up to the observation deck of the Sears Tower, shouted some appropriate Arabic, and pushed the button- a huge nothing happens, and he goes to report back to his cell leader.
For amusement,
http://www.osti.gov/historicalfilms/opentext/data/0800031.html
This video clip shows a Seal delivering a nuke- he jumps out of a helo and parachutes down into a harbor with a 10 kt bomb strapped to his belt.