A contract can have a mortgage contingency date - where if you do not qualify and you notify the seller that you do not qualify during the dates specified in the contract your deposit is protected.
A contract can have an inspection contingency date - where after conducting a home inspection by a licensed home inspector and the house is not up to par and you are within the dates specified in the contract your deposit is protected.
A contract can have an appraisal rider - which would protect your deposit during the dates specified in the contract your if the home did not appraise.
A contract can have a home to sell rider (also called a Hubbard clause) it may have an Attorney Approval clause or Well / Septic contingency...Long story short read your contract and know your dates. If you are within your dates specified in the contract - then the sellers must also follow the contract. If you are not within your dates - call your realtor and your real estate attorney TODAY!
Best of luck!