Be selective and critical. Talk to other people: Before reading, ask, "Is it worth reading? All of it? What parts?" Whilst reading and after completion, discuss the content with others for further clarification and better understanding.
Techniques that work for some: knowledge & information mapping (graphs, brainstorms etc), highlight key words/points, paraphrase sentences, compare with other related work, compose a critical appraisal.
Some tips: Set specific goals, schedule realistic work spells, read at the right time, read in the same place, read from paper rather than a screen, use your knowledge map.
Some possible reasons: Sub-vocalisation, regression & backskipping, finger-pointing, interruptions, low light & discomfort, fatigue, poor vocabulary/comprehension.