They can have uneven germination. The first seeds I ever pinched from a public place were African Iris. One half of the seeds came up within weeks of sowing, the other half came up sporadically the following Spring and Summer in the garden bed I had dumped the 'dud' seeds into after two months of waiting. This was before I learned about stratification or scarification, so I imagine one or both played a part in the seeds that germinated later.

If you want a few good clumps just baby them through spring. By summer those individual plants will be large and in charge and spreading fast with just a little care and some fertilizer. :-)