I hae a lot of koi, but only 3 (now only 2, 'cause one jumped out and died) are females. I guess I just like the colors on the males. So, it is possible to end up with all the same sex.
If yours were spawning, I doubt you'd miss it... they get so fat for the days prior, and spawning takes about the entire day, you would surely notice SOMEthing was up. Sounds like you simply have all the same sex.