Hey, Rich. You're not the only one who feels this way. I've not exactly played for anywhere near as long as you, but in my now 2 years of playing, I've found that some riffs and licks come much easier than others. Generally, to me it seems that slow, melodic parts of solos come easier than faster shred-parts (might be that it's because of my lacking speed, but it might also just be that I have an easier time memorizing slower, melodic parts...). In fact, it only took me two attempts to learn the lead parts for "Without You" by Mötley Crüe perfectly, and 4 or 5 attempts to learn "Knockin' On Heaven's Door" as played by Guns N' Roses. On the other hand, anything that involves Arpeggios or stuff like "Eat The Rich" or "Livin' On The Edge" (both by Aerosmith) just keeps avoiding me, no matter how hard I try!

It's annoying, but I intend to get those faster tunes that I'd actually enjoy playing Live into my finger-tips, no matter how long I'll have to practice to get them there :)

Just keep going at it, and you won't have much to worry about :)