If you can't play in the first position, but can up the FB, its usually action at the nut. Unless, something else is off, or not suited to you - neck shape, FB width, fret size, etc.
Putting a smaller guage string on a larger instrument would usually result in lower tension, resulting in lower volume, etc, and the potential for buzzing. But, without seeing the instrument, hard to tell.
Valerie, as for strings, you asked:
I don't use Worth - too thin for me, although I have it on my Concert KoAloha (that my 6 year old plays), and its fine.
I don't care for Aquila. Yes, they are loud, but lack complexity. They also intonate off, and smash if not careful. I prefer Hilo's over Aquila, at least from a consistency standpoint.
Used the old Ko`olau, liked the sound and tension, didn't like the feel, but haven't used the new ones.
Personally like:
D'Addario Pro Arte J46 to get a sweeter sound, usually prefer the J50 to get a brighter sound. Depending on the bracing, I'll also use the J45 and J49 series (lean towards J49) for lighter braced instruments. One drawback for most - these are larger guaged, so the nut has to be recut, but no bid deal. The high tension strings are really nice on Spruce tops, again, with the nod to the J50 series - not too many good black nylon strings around, and these are the best for me.
Usually install:
Savarez Alliance 540 Series R & J, again depending on the bracing. These are about 3x the cost of D'Addario, but who cares, its only $12 for 4 strings. Again, the nod goes to the higher tension usually, so the 540J's. These high tensions are not as stiff nor thick as the D'Addario's, and are more forgiving on the fingers. Bright and clean, not as loud as the J50's, but great overall. Composition is Carbon (similar to Worth, except thicker, and definitely not tinny), with a very balanced tone.
By the way, you'll notice the D'Addario and Savarez are not packaged `ukulele strings, they are classical guitar strings. The D'Addario J46 is what their `ukulele J71 (I think) set is made from for Jake. As of January, I met with the Savarez rep, and he is working with his American distributors to come up with `ukulele sets. They have the strings right now to repackage for Tenors, but I think their target market is the mainland, so more Standard and Concert rather than Tenors.