| | Re: pcr smear
PCR smears can be caused due to a number of reasons. Template inhibition is one of the reasons, as stated by danfive. When conducting reamplification from a previous product, it is possible to get short smears. Reducing template concentration in serial dilutions might solve the problem. Also, reducing the number of cycles could help; reducing the total amplification also will reduce the non-specific amplification.
Also remember, the same PCR reaction will behave differently from well to well, heating block to heating block, PCR machine to PCR machine and PCR make to PCR make. So keep those things in mind while optimizing your reaction.
When countering PCR contamination, also a reason for smears, try those reagents which are introduced in larger volumes first. Water is an important source of contamination. Change the PCR buffer, MgCl2 solutions and try again. Always remember to thaw-vortex your reagents well before use, and remember to change one variable at a time.