Historical sources and methods are targeted at answering contextual questions about the meaning and nature of human behaviour and experience over time on the basis of the documentary record of the past.
Questions must be timely: they must deal with change and continuity. They must deal with the situation in time.
Questions must deal with the documentary record of the past: history is fundamentally a textual pursuit. Even when historians construct, for example, wage-price series in economic history they are reliant upon the textual records of the past. This question can become problematic when the documentary records of the past are recent in origin (for example: evidence supplied by archaeologists).
Questions must deal in human practices and behaviours: while we can observe evidence for climatologists or botanists from the documentary record of the past, these activities then become biology or ecology. History itself is concerned with the human world of meanings and experiences. Many things which may not appear to initially be human meaning or experience (boat construction) actually are: the science, craft and practice of building boats is an intensely social and meaning driven endeavour.
Questions must deal in context and meaning: Historians typically answer questions about meaning, rather than volume. While answers may be available for questions such as "How many soldiers were in a division in Germany?" a historian will seek to answer, "Why did Germans choose a certain divisional structure?" As such, many things that people wish to know about the past cannot be answered by historians, as the questions are not meaningful or contextual: a practice may be an irrelevancy, or the sources may not record the practice in a way (or at all) that allows it to be contextualised.
This is because historians deal with the documentary record of the past, which tends to be documents that people cared about preserving (or did not seek to deliberately destroy). These records tend to be about the meaning of human social practices. Additionally, as historians read meaning from texts, the methods of reading meaning produce limitations on the answerable questions. The use of empathy or historical economic statistics reduce the number of produceable meanings.
Thirdly, this is because mostly people want meaning and context questions about the past. "What was it like to be...?" rather than "How many nails in a hob nailed boot on average in 14th century Florence?" and the funding agencies support the former but not the latter question.