Some lucky March applicants already got CO....it is no brainer that immi use this in an effort to stop people calling asking for CO. If you do not have CO, all you need to do is to look at the cut-off date at which ALL applicants already got CO allocated. Of course the date is not always reflect real-time allocation as AK already had case officer or some other early March applicants.
The delay email is very common since the current program year is close to an end and quota for 190 is nearly used up. There are some places left and there is a chance you might get grant before end of June, your CO wouldn't know when for sure that is why he/she cannot answer you confidently. If you do not get grant this year, then the new program year kicks in 1st July and 190 starts fresh with new quota, it is HIGHLY LIKELY you will get your grant then. DIBP has strict planning for each year how many migrants are coming to Australia, therefore you will not be granted this year if they runs out of quota for 190.
Don't worry, sooner or later you will get grant!!!! Hope this helps.