The clutches are engaged using hydraulic pressure. The input shaft from the engine to the transmission runs the hydraulic pump which causes EVERYTHING to happen inside the transmission. A section of the transmission called the valve body is responsible for routing pressurized oil to engage the different clutches. Any point that results in low hydraulic pressure will cause the clutches to slip or fail to completely engage. Low hydraulic pressure can be caused by: low fluid level, a worn hydraulic pump, leaking seals in the valve body or even a partially blocked transmission fluid filter. Unfortunately, a blocked filter usually means clutch debris has built up to block the filter which causes the clutches to wear rapidly. Bottom line, once your transmission reaches the point you describe, it's probably done. If it's caused by anything other than low fluid, you need to either rebuild or replace the transmission. Even if it's low fluid the transmission could be permanantly damaged. I wish I could offer better future.