It's not a big deal really...mine do this all the time and I just prop them up with bamboo skewers or something similar for a few weeks until they get their stems caught up with the rest of the plant. There are several ways to strengthen stems. Best way IMHO is to keep a very small slow blowing fan on them indirectly...just so they barely move. I don't really know about this other technique, but I've read if you gently shake the stem base for about 10-20 seconds each day, the plant will mistake it for "wind" and work to strengthen the stems...however, I'm not brave enough to do that to my 2-3 day old sprouts! There may be other ways as well, but I'm ripped right now and can't remember! DOH! Hope this helps! PH shouldn't be an issue, because you have that exactly right all along, right?????