Is percent yield always less than 100?
In theory, no. In practice, yes.
If you were able to react all your raw material, and if only the reaction you wanted took place, and if you could recover all of the product you made without any losses during recovery or purification you would have 100% yield. In reality, not all of the reactant reacts; other side-reactions take place creating unexpected or undesirable products, and the process of removing the wanted product from the reaction vessel and purifying it causes further losses so the percentage yield is not perfect.