How close to the radiator? Over 80 degrees and you start having to worry about fungus.

Some lines produce seeds with a very thick shell that is often slow or difficult to crack open. Thos seeds could use a light scuff along the sharper edge with a nail file just to give them a tiny bit of advantage.

Old seeds are also slower to germinate or have a lower % germ rate. Give up to 2 weeks.

The rule of thumb is seeds are considered 'good' within 5 years of their harvest date. After that, germ rate may decrease by about 10% per year.