I collected the seeds of many heirloom tomatoes from the US. The trouble with these seeds is that they are not yet acclimatised to the terroir here. These are already two weeks and only half have sprouted, and they are growing very slowly.
The same thing happened last year, even worse actually. Out of four varietals, only one type, green zebra, managed to grow, rather poorly, and bore only one tomato. Just one tomato.