A woman wrote to me asking what if anything could be done to compel the Plaintiff's expert witness to testify; the expert already hired and the suit filed appeared to be having a case of cold feet and was no longer available for the trial.
What she describes is a problem that has no simple and workable solution. A party can offer the expert a deposition as an alternative. But if she's got cold feet and has had a change of heart about testifying, then your case is in trouble. If you subpoena the expert you may not like the testimony, as it may be useless in terms of proving your case.
You could sue the expert for costs associated with litigation preparation. Is there any money on the table? You could settle.