Why Has My Stake Dropped Out?
Each validator has 1,000 slots for delegated stakes. Each separate coin takes up its own slot, while units of the same coin are summed and share a common slot (for example, if you've delegated 2 BIP and 10 TEAM, and then 10 TEAM more, you'll take up two slots with the stakes of 2 BIP and 20 TEAM).
To get in, your stake has to rank among the top 1,000 by volume. If a given validator has at least one vacant slot, you can delegate any amount.
If all 1,000 slots are occupied, then in order to delegate to such a validator, you need to make sure that the amount of BIP (or custom coin's BIP equivalent) you're about to delegate is greater than the minimum stake which takes up the last slot.
If your stake has been squeezed out of the 1,000 slots, your coins get waitlisted. If that happens, you'll need to delegate more coins, or unbond your stake entirely, or wait until the minimum stake drops to the level you're comfortable with and additionally delegate any amount big enough for your stake to outnumber the one currently ranked last.
To monitor how your stake ranks against others, visit given validator's page at Chainik.io/validators. Alternatively, you may track the page of your address there as well.