Looks like the theoretical limit to suck water is about 34 feet, or in other words one atmosphere (because the atmospheric pressure is what is pushing the water up the pipe if we could create a perfect vacuum within the pipe).
You are probably no where near that, but you may be far enough it is hard or near impossible to get enough vacuum to get the flow started. The slightest air pocket could be a problem.
lifting water A vacuum pump at the highest point in the pipe with proper valving might be the way to go.
