Here's my basic understanding of this, but as I'm also new to all this PoS business, I could be way off. Someone correct me if I'm wrong.

All normal, mature coins are "Stakeable".
Coins in "Stake" are coins that recently received PoS coins. It creates a new output that needs to mature again, that's why it isn't spendable. Once it gets spendable (mature) again, it shows at the spendable balance and will start to attract stake again.

So for example, you have 1 address, where you got 3 transactions.
1: 10 MAST
2: 100 MAST
3: 1000 MAST

When these coins are mature, they start trying to find PoS blocks, each output individually. So you have 1110 mature coins under "Spendable". The 1000 MAST output has a higher chance to get stake reward than the 10 MAST. But over time, the chance increases, so that the 10 MAST will eventually also find it. When the 1000 MAST found reward (say 0.1 MAST), you'll have a new 1000.1 MAST output that needs to mature (1200 confirmations) again. The 1000.1 MAST then show up under "Stake" and can not be spend (or receive PoS reward) until they mature. The remaining 110 are still under "Spendable" and waiting to get their PoS.

About the "'generated but not accepted' (i.e. looks dead)": That usually means those PoS blocks you mined are orphans, so the network has decided to take another block instead of yours (happens when blocks are found at almost the same time by different people).