By design, a synchronizer is used as a cone brake to match the speeds of various components inside the transmission so that they are "synchronized" in relation to the speed that the parts are moving.  When this is being done correctly the transmission will shift without any grind or crunch as you complete the shift. 

Also by design, a synchronizer will block the driver from completing their shift into any gear, and for that matter, even make shifting into any gear in the first place difficult if the speed of the clutch disc, input shaft & various other components doesn’t match the speed of the gear they are shifting into. The synchronizers are designed to block the shift for the reason that if they didn't it would just crash the engagement teeth on the sleeve and on the gear which will lead to lots of damage and the shifting would get worse and worse till it won't even want to stay in gear.

Synchronizers are not designed or capable of dealing with a clutch that doesn’t have enough release.  If you are having trouble getting into any gears while the car is running or are experiencing blocking or “notchy” shifting; you must check your clutch, making sure that it's properly adjusted or you will cause damage to the synchronizers. It is sole responsibility of the driver and/or installer to make sure that the clutch is properly adjusted so that is has sufficient release to allow the synchros to do their job without the clutch disc dragging on the flywheel/pressure plate assembly. Understand that the synchronizers are not capable of dealing with the task of trying to slow down the engine to the synchronized speed of the transmission and oftentimes the synchronizer ends up getting damaged by the driver who is hell bent on stuffing it into gear and they will need replacement.

It is crucial that your clutch is operating properly, with sufficient release to allow the clutch disc, input shaft & various other components to be “free” from the engine’s rotating speed thus allowing them to slow down and match the speed of the gear you are shifting into.

Transmission synchronizers, like brake pads, will require some time for their surfaces to fully bed-in. Do not shift a new or freshly rebuilt transmission fast or at high rpms for this bed-in process requires that you drive the car gently and shift normally, without excessive force. Understand that you want to allow the synchronizers a little extra time to develop their full contact finish which provides the most friction and ultimately the best shift quality. This process takes a few hundred miles of city driving, where you are shifting through the gears often. Rush this process by shifting the hell out of it and your transmission will not shift as well as it could have. Take your time and fully read this page.

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