各位老师求解答：
        我采用相同的数据和处理方法，结果有时候能够正常回归，有时候确没有t检验结果，请问这是为什么呢？
         代码reghdfe Y did ,absorb(year 行业 省份) vce(cl year 行业 省份)
         异常结果提醒：
Warning: VCV matrix was non-positive semi-definite; adjustment from Cameron, Gelbach & Miller applied.
warning: missing F statistic; dropped variables due to collinearity or too few clusters