Hi Jackie,
 
Thank you for your enquiry.
 
Without knowledge of the actual model you have, what type of base it is on and what your body weight(s) are it is not possible for us to provide an opinion as to the cause. Having said that, if the base has remained constant, it may be that this is failing.
 
If you would like to call our office on 0161 437 4419, we will endeavour to assist further.
 
Kind regards.