This can be caused by the waste water pipe getting clogged up with conditioner and hair/fluff etc. There are sensors in the machine that won't allow it to spin if there is any water in the drum, sometimes a partly clogged pipe won't allow the drum to fully pump out before the timer moves on
to the spin cycle.
UNPLUG THE MACHINE before working on it, don't just turn it off at the switch. Disconnect the waste water pipe where it leaves the washing machine casing, usually the lowest point at the back and remove any gunge gently with a screwdriver (in the pipes both inside and outside of the machine, the pipe clips are usually reusuable and will need some pliers to get them off/on). Make sure you have a bowl under the pipe at all times (best turn off and disconnect the incoming water supply as well just in case). Reconnect the hose(s) and try it out by advancing the timer to just before the spin cycle. Check for any possible leaks before doing a load of washing.
This type of conditioner clogging can be caused if you mainly do low temperature washes.
Put half a bag of soda crystals in the drum once a month and run the machine empty on it's hottest wash cycle, this helps to keep the innards clean.