The usual cause for this is either:

A).  You dried it at too high a temperature.  We smoke (dry) our jerky at a maximum temperature of 180°.

If possible, place an oven safe thermometer in your oven, set the oven to the lowest temperature possible, and see what that temperature is.  If it’s quite a bit above 180°, that could be the problem of the jerky being too dry.  

B).  You dried it for too long of time.  We usually dry our jerky at 180° for 6 hours.  This usually produces jerky that is dried, but not too dry.  You want the jerky to be able to bend without breaking.

I hope one of the two above suggestions helps you with your jerky making.