Dogs must always enter the weaves with their left shoulder through poles 1-2 as per the diagram. In Starters, there are no refusals called on weave poles which gives beginner dogs the opportunity to attempt the weave poles as many times as needed to get the job done. As you progress up the levels of difficulty (Advanced, Masters), refusals will be called on weave poles. This means that if the dog runs past the entry (poles 1-2), goes in "backwards" with their right shoulder, pops out of the poles somewhere along the way without completing the set, skips poles, or enters the poles at any point other than poles 1-2, you will be faulted (which means your run is no longer free from mistakes). 

The most difficult part for the dog is the entry (poles 1-2) and the subsequent "bend back" (poles 2-3) as they have to calculate how much they need to collect their stride to be able to get into the rhythm of weaving the set of poles. The 1-2 obstacles before the set of weave poles and the angle of approach are both critical in determining how successful the dog will be for their entry and bend-back.

Speed into the Weaves

When the preceding obstacle is something that provides the dog with speed and extension (like a straight tunnel or broad jump), the amount of collection (deceleration) required for the dog to make the weave entry and the subsequent bend back is much greater.  

Distraction into the weaves

Missing a weave entry can occur for many reasons, but i can be due to the some sort of visual confusion. This happens to many dogs when weave poles are positioned along the edge of the ring and the dog sees the fence posts, mistaking them for weaves!