should ap- ply Rule 9(b) with some flexibility and should not require plaintiffs to plead issues that may have been concealed by the de- fendants. Rolo I. City Investing Co. Liq- uidating Trust, 155 F.3d 644 (3d Cir.1998). Although the rule does not require a reci- tation of "every material detail" of the alleged fraud,