In terms of importance, starting material is key. Sufficient amounts of RNA need to be enriched to detect proteins. Hence, target RNAs that are lowly abundant may require the analysis of large numbers of cells which can pose challenges in terms of cell culture storage and handling capacity. Other notable factors that influence enrichment efficiency include cross-linking and sonication conditions, salt concentrations in the buffer, hybridization time (overnight hybridization has been reported to vastly improve RNA enrichment), and probe concentrations. Useful blogpost: Tips for optimizing RNA affinity purification.