A very sad event indeed and my sympathy goes to the loved ones of young Tia.
The investigation into this matter is in the very early stages so to attempt to draw any conclusions at this point is inadvisable, and for the most part, speculative.
Police action in such cases will always be criticised but it should be remembered that few people outside the prime investigatory team have all the facts available to them – even other police officers – so the opportunity for others to get a global perspective of the state of the investigation is quite limited indeed.
On the general point of not finding bodies when there appears to be few reasons why they shouldn’t be found, the only thing I can say - having had my fair share of finds after interminable searches - is that in the absence of the unforgettable odour of decomposing flesh, it's remarkable just how many places can be used to secret a body - even within the confines of a modest domestic property.
The moment for an intensive search will always be fluid and will generally become obvious when certain facts come together; a premature search can sometimes detract from a particular investigatory line.
In my experience the police were following just such a line in this particular investigation and, having allowed certain conditions to develop, the (sadly) logical conclusion to the initial stages led to the confirmation that this unfortunate child was indeed deceased.