When editing an activity that includes collecting objects, (or avoiding objects), you can alter the sensitivity of the collision detector.
Within a platform activity if you right click on the "test activity" (green triangle) you can alter the following code;
_root.hitDist=40;
The smaller the value, the closer two objects must be before they interact with each other.
Note however that if you set the value to "0", you will not be able to collect any items.