Assuming a reasonably efficient market, you can expect the returns from mining to be (on average) about equal to the cost of mining (which is mostly just electricity costs). This means that profit margins are razor thin and only the most efficient miners can make a lot of money. Mining is not a get-rich-quick scheme. It is a business, in which you provide security for the Bitcoin network, and get paid based on results. It is a highly competitive business, and if you can't mine as efficiently as your competitors, you won't make a profit. It's as simple as that.

There are no guarantees with solo mining, it all depends on luck. Every hash you calculate has a (very small) chance of meeting the "target". There is no way to know, apart from actually calculating the hash, whether it will meet the target or not. The more hashes you produce, the better your chances are that one of them will meet the target. This element of chance with solo mining is the whole reason pooled mining exists.