Propagation: What's going on with my portulaca seedlings?, 0 by LAS14

LAS14 wrote:
I planted portulaca seedlings in mid April. Here it is over 2 months later, and this is all the bigger they are. Is this normal???? I planted them in windowboxes mostly filled with professional compost (bags branded by our local garden center). But I put seed starter mix on the top two inches. I think over time the seed starter washed away. I put them out in the sun as soon as they sprouted and only brought them in when cold threatened.

Anyway, they seem to be going in slow motion and don't look like they'll be ready to plant outside this season. Anything I can do differently next year?

TIA
LAS