This is my guitar:

These are my new strings:

And the problem is, the first 3 strings, E A D, all sound fine, because they're coiled, but the higher 3, G B E, all sound flat, especially when playing chords. I'll be watching this thread, so by all means ask questions, and I'll try to answer them. But any help would be appreciated.
“You have enemies? Good. That means you've stood up for something, sometime in your life.” -Winston Churchill
Typically, nylon strings take a while to self-stretch until they hold their tune properly. A few days is not unheard of. Whatever you do, don't help them along by pre-stretching them as you're installing them. This will create thin spots in the string that will screw it up. Nylons are best installed on the guitar, then tuned up near to pitch, play on the guitar a while, retune then let it sit overnight. Retune, play, retune,let sit. After a few days of this routine the strings will have done their thing and should start holding their tune nicely. Once they're steady, you'll notice that you will start getting more volume and tone out of them as well. They're odd strings to me, since I'm an avid steel string guitarist, but have played and serviced classicals also. They just need a bit more TLC in the beginning than steels do.

Hope this helps.