I read this but wondered if his initial question wasn't more about sustain than strings in general.
Is your initial question about how when you hit a string it rings loudly and then fades too quickly for your liking? That's sustain.
If that's your question, then a fairly short sustain is just part of the nature of the instrument itself. Certainly depending on the builder and the strings you're going to get more or less sound and sustain, but given the short scale length for the strings and the limited size of the soundboard (the top of the uke) and how nylon strings are, there's just going to be less sustained vibration over a shorter period of time. The sound is just going to run out of gas a lot faster with everything being smaller and shorter. The ukulele is just not an instrument designed to play ringing power chords.
Certainly if the strings on your instrument are already a couple of years old they're not likely to have much left in them. Changing strings will help.
As it is, you have a very nice uke.