Sometimes if LW is overworked it seems to stop exporting to LW. Best thing to do in that case is close LW & re-open it. For the songs that didn't import then just drag & drop onto iTunes in your dock!

Just be aware that iTunes doesn't import all types of audio. mp3 Pro & Ogg Vorbis. But you can get xtra iTunes plug-ins .. & at least to import Ogg Vorbis. For mp3 pro get Audion whilst it's still available & now for free. It has more encoding & decoding options. See: Issues you should know about iTunes & iPod