If AVE is below than .5, then your DV also will not fulfill the criteria of minimum value .70 because square root of less than .5 may not lead to find DV greater than .70, So you can check again the factor loading and can drop items which creates problems if you have enough items (minimum 3 in AMOS)
You drop the item with lowest factor loading from you construct and recompute the AVE. Remember - AVE below 0.5 means you construct does not have Convergent Validity and also failed Construct Reliability.
Sometimes due to confusion respondents may give wrong opinion for a particular item. If there is strong theoretical support then go ahead for further data analysis