Why do some combinations of ionic compounds form a precipitate while others do not

Whether or not such a reaction occurs can be determined by using the solubility rules for common ionic solids.

tell me if i'm wrong

This is because of the solubility of the reacting metal salts.

Explanation:

The use of solubility of salts influences the formation of precipitates. Most precipitation reactions occur as a result of the replacement reactions or double placement reactions. A double replacement reaction occurs when two ionic reactants dissociate and bond with respective anion and cation of the other reactant. This is simply a switch over reaction.

A double replacement reaction occurs when a chemical reaction occurs and one of the products formed is insoluble.

Here is an example:

Both reactants are acqeous and the product is solid.