Please post the specifics of this system, for a better diagnosis and suggestions.
Memory, if installed as one module, must be in the first-detected slot, typically the "bank 0, slot 0" position. Some oddballs require a different option. Best to read the user's manual. For very old boards with SIMM modules, many required two modules in the 72-pin or four modules in the 30-pin versions. Modern DDR memory requires only one module, if a single-channel setup. Can also be applied to multi-channel setups, for testing.
As to older systems, they may recognize only single-density memory setups. High-density modules would show only 1/2 the true module capacity, on the POST pre-boot screen. Whether it be single-sided or double-sided modules on a memory card, does not indicate the single-density or high-density status. The logo on the chips and the module tag have the info for research.. and usually the status.
When mixing modules of different speeds, it is important to put the slowest module in the first-detected slot. This sets the speed for all modules. Faster modules will clock back with good stability, but slower modules will falter at higher speeds. With multi-channel memory setups, you must have the identical memory amounts in all channels.
F@H.. to solve mankind's maladies.. in our lifetimes!