Rm pumps depend on velocity of the water to work. The faster the water moves, the more/higher you can pump it.
If you have slow water without much head one possible alternative idea I've seen is to use a teeter-totter type pump. This only needs 1-2 feet of head. The water flows into a '
bucket' one one side until it's full and gravity tips the arm down, when one side goes down the water is directed into the other side and starts to fill it, meanwhile the bucket that is down starts to empty. The tipping back and forth drives a standard reciprocating pump to move the water.
The larger the buckets, the more force you get, but the slower it pumps.
It doesn't have the elegance and simplicity of a ram pump, but it does have the advantage of being able to work with very low head and very little flow.