Yes, as Dravis says, more detail is required in order for us to diagnose your problem. As it happens, Dravis has already pointed out the most likely cause which is the previous layer having cured too far (past its B stage) in order for the new layer to cross link with it chemically - this means you're reliant on mechanical bond which then means that surface prep (keyed surface) is essential. Provide as much info you can as to your process, other materials involved, work environment and timings and we should be able to put a finger on the issue. When used right the inter-laminar bond between layers of XCR should be excellent but it is a fast curing resin and if you miss the window then the bond strength will be negligible without proper prep.