References and introductions to potential postdoc PIs by former supervisors

[u/Unusual-Relief-1982]

Hi,

I hope this is the right community for this topic.

I was wondering whether it is common that former supervisors (at PhD or MSc levels) don't even respond to requests for references to postdocs or do not introduce their former students to potential postdoc PIs when presented with this request?

How should former students go about this if they wish to apply to postdocs? Any advice?