Make sure your burning them in the same region i dont know where your from but u know which region no u are right? The downloads will not all be in the same region. It does take a little longer if your converting region for nero to convert upto twice the lenth of time. If this doesnt fix it try a lazer cleaner on ur burning pc, no good? try a different brand of blank disc they may be poor quality. If all this doesnt work then my final tip at this stage would be to lenthen your burning time doin a double converter pass instead of single and also slow down the actual burning this makes so many peoples problems just dissapear.