Rising (1) - S1-E1
Continuity mistake: When they first arrive on Teyla's planet, the DHD is close to the gate. When the wraith attack, the DHD is much further from the gate.
Continuity mistake: Right at the end, after the entity has gone through the gate, we get an overhead shot of McKay lying on the ground in front of the gate. His legs are stretched out straight. In the next shot, one of his legs is bent. He's unconscious, so he shouldn't have moved.
Continuity mistake: When they finally get through the gate by blowing the rear hatch (which serves as the exit ramp when open), they arrive on the other end with the ramp closed.
Continuity mistake: Right at the beginning, the crew of the jumper only dials five symbols into the DHD before hitting the engage button.
Continuity mistake: In the scene where the Wraith attack on the Mainland, Sheppard is stunned and falls with his right hand on his gun near his hip, but when McKay and the other guy grab him, to pull him to the gate, both the gun and his right arm are above his head.
Continuity mistake: Before Merell enters the cage to be fed on by the Wraith, his collar is open. Next shot when he enters the cage the collar is fully buttoned. (00:28:08)
Chosen answer: Because the ZPM allowing them to dial Atlantis was being used at the Antarctic defense outpost. They couldn't risk making Earth vulnerable to attack from Anubis by depleting it with repeated dialing to the Pegasus galaxy. Once they knew Atlantis was in serious danger, they had no choice.
Grumpy Scot
ZPM from Antarctic was depleted in first dial, dial in Siege was done via ZPM from Egypt (SG-1: Moebius Part-1/2).