I planted some bamboo seeds in potting soil and when I pour water on it
the water just stays on top. If I let it soak in, it only goes like 1/2"
down. Do I have to let it soak in longer?

Put the whole pot in tepid (baby-bath-temperature) water, up to the rim,
overnight. Allow to drain. Keep the soil moist or you'll probably have to
do it again.

An alternative can be to add a small amount of surfactant to the water and
top water. In the past, I've found one drop of shampoo per quart of water
satisfactory.

Next time you use this potting soil, throw some hot water in the bag a couple
days before hand, close the bag, and knead it a bit... then leave for 48 hours
or so before planting. It'll probably wet easier.

Kay