There are several reasons why an engine might crank slowly, then not start:

1. Weak battery or bad connection - Your car battery may be low on charge or have a bad connection, preventing the engine from starting.

2. Faulty starter motor - A bad starter motor may intermittently turn the engine slowly, leading to slow cranking.

3. Clogged or dirty fuel filter - If the fuel filter is clogged or dirty, it can reduce fuel pressure and prevent the engine from starting.

4. Insufficient fuel pressure - A malfunctioning fuel pump or a fuel pressure regulator can cause insufficient fuel pressure to reach the engine, leading to slow cranking and a no-start condition.

5. Ignition system problems - Worn spark plugs, a faulty ignition coil, or a malfunctioning crankshaft position sensor can cause slow cranking and prevent the engine from starting.

6. Timing belt or chain issues - A worn or loose timing belt or chain can cause the engine to crank slowly or not at all.

7. Bad alternator - If the alternator is failing, it may not be able to supply enough power to the battery, which can result in slow cranking and a no-start condition.

It is recommended to have your car diagnosed by a professional mechanic to determine the exact cause of the problem.