Hi,
to the forum!
Is the Celestron 127 a reflector or refractor? What is it mounted on? I guess 127mm is the size of the objective? What's that, 5"? It must be a 5" reflector then?
Have you checked collimation? What time of night did you view it? How high above the horizon was it?
You're really missing something like a 10mm eyepiece for that scope - the 20mm is a low magnification. Jupiter's disc would be quite small, but if it's well focused and collimated, you should still be able to see the 2 main belts. If it's too bright, you could try a filter or try increasing the magnification.
Unfortunately with the 3x barlow (likely to be of questionable quality), when used with the 20mm, it gives you approx 6.5mm eyepiece, which is probably too high power for this scope, especially if the seeing conditions aren't very good (which they haven't been, lately) and if the scope is not well collimated.
Something around a 10mm eyepiece (if you had a 2x barlow) would be better, as the higher magnification would make it appear less bright, and hopefully the 2 brown belts would stand out a bit clearer.
To get you by, you could get a cheap 2x GSO barlow from somewhere like Bintel or Andrews. The 4mm is pretty much junk, you may as well throw it away as the likelihood of it a) being good quality, and b) seeing being good enough to use it, is extremely unlikely.
Hope that helps.