I'm building my first wheel. It's a rear wheel. I got the spokes at the LBS, they measured the hub and rim for me to figure out the correct lengths. I've also confirmed their measurements using the DT Swiss calculator and manufacturer's specs.

I followed Sheldon Brown's guide, have read through Jobst Brant's book, and watched some videos on Youtube. I'm fairly confident that I got the lacing pattern right.

Now, the problem: It seems that all of the spokes going one way (trailing) are quite loose after lacing, while the leading spokes are fairly tight. The last few leading spokes were a struggle to get into the nipples.

Is this normal?

Secondly, I thought the problem would resolve itself as I started the initial tensioning. While I was doing this, a few spokes popped out of their nipples. They would have only been in by a few threads. Should I replace those nipples before I try to correct this problem?