This may be due to a couple of reasons:
Your LED bulbs may not be dimmable. Not all LED light bulbs have the ability to dim so you have to make sure that your bulbs are the dimmable type.
Ensure that you have an LED-compatible dimmer switch. Standard household dimmer switches do not work with LED bulbs. The minimum wattage figure on the dimmer switch is too high; typically the majority of leading edge dimmer switches are 60 watts which is way too high for operation with dimmable LED bulbs. This is why you need an LED compatible dimmer switch for use with dimmable LED bulbs.
To ensure free flickering, please use silicon-controlled rectifier (SRC) type of dimmer with dimmable bulbs of LE.