Legislative assemblies have developed a variety of arcane procedures often meant to stifle dissent and smooth things for the majority. In the House, no big deal because it was designed to be contentious and subject to majoritarian whims, but in the Senate these processes have been argued to negate the intent that the assembly allow minority interests to influence the process.
Over the past two decades Senate majority leaders have developed a process called "filling the tree" which prevents the minority party from adding amendments to a bill by filling up available amendment slots before the minority has the chance to offer any.
Robert Novak is critical of the current majority leader's use of the procedure and calls it a new low in Senate history. The Senate is no longer the world's greatest deliberative body as it has come to call itself. It is worth speculating about whether this has in fact ever been the case. Here's a good read along these lines from a man calling himself The Decembrist.
