It may just be normal senescence of older leaves. To test, you could carefully clip the old, yellow leaves from one stem and see if that enables/encourages more branches to grow where those leaves were. The tips look fine.
Creeping plants that would normally take root at every node as they creep along the surface can tend to lose some vigor when they've crept over the edge of a pot and can't keep making more roots. To keep things fresh, you can periodically trim a vigorous tip on a stem that has lost older leaves and make a hole about an inch deep with a small nail or similar object, and stick the stem in the hole to take root and keep growing.