a.  Lack of knowledge, experience or technical skill

b.  Lack of time

c.  Insufficent capital

d.  Lack of discipline or systematic approach

e.  Lack of tools/data

f.  Failure to properly test a trading system. Ex: over-optimization, bias, etc.

g.  Failure to monitor your results (poor organization)

h.  Inadequate risk management

i.  Too few trading opportunities

j.  Other (please specify in thread)