Family visit visa delay

[Twinkleeyes]

I have invited my mother to visit me for a month. She handed in her application on 18/05/2018 in yaounde -cameroon. Recieved confirmation and being prepared for consideration since 24/05/18. All i have recieved from sheffiled is an email stating that the application is complicated and would take longer than stated time. We are still awaiting a decision and have contacted UKVI twice at a charge of £5.38 each time. The reponse has been something like 'we trying to process application and cannot give you a time when decision would be made'. Is this normal? Why is it taking so long??