Other people have given the technical answers for chain limitations, but I am wondering why you feel like you are at a two-minute song length from a musical perspective (ignoring BPM and adjusting available steps)?
If you take one track, set the step count to 16, set the step length to 9 (I think this is x16 multiplier?), then you are able to have 256 notes (triggers, noises, whatever) per pattern. Someone correct me if I am wrong about this.
That is for one pattern, and you can record 16 different patterns, and then you can make up to 32 chains of them. So if I had patterns A, B, C I could chain…
AAAA BBBB AAAA BBBB ABAB CCCC CCCC ABCA before it would have to loop back. I usually only use 4-10 chains (AA BB ABC) while using the tape and performance effects and that will create several minutes of non-repetitive music.
So to get 15 patterns to only last two minutes, are you chaining them like ABCDEFGHIJKLMNO ABCDEFGHIJKLMNO and having the music change every three seconds? Are you using step components or performance effects?