Are you doing the DNA isolation with a commercial kit? If not and you are using a protocol. Have you prepared any new reagent again? It sounds that the last extractions you did where done a while ago. If you prepared some reagent again maybe that can be the problem.

The other problem can be the tissue. Are you collecting and using the leaves to do the isolation in the same way? I don´t know maybe you collected and the preserved in the freezer. Some plants don't like that. I you are doing exactly the same, can be as Stalis suggest.

First let me What method you had been used and facing problem in where exactly,then I can suggest you precisely or else you can follow CTAB method thats very selective for DNA Extraction from Plant source.

you are using DNA extraction kit? i thing problem in lysis of cell wall. because plant cell wall is much harder than bacterial cell wall so u can check your lysis buffer or add more amount of lysis buffer.

Please describe any steps your method

CTAB is the best protocol in any way. And yes i agree with maimuthu that it seems that problem is in cell lysis. perfrom your cell lysis step efficiently. you will definitely get dna. Good luck

First of all grind the leaves properly.

CTAB method is good for isolation of DNA.

Check the pH of all the reagents used. pH of the reagents is major concern in DNA isolation.

precipitate the DNA carefully according to the protocol.

Try with the some one other's reagents.

May be helpfull...good luck

"CTAB method for DNA extraction protocol"

or "CTAB method for DNA extraction protocol from herbarium"

http://primerdigital.com/dna.html

Please tell us what protocol you are using.

CTAB is a good one indeed.

Remember:

1. Even if your lysis/homogenization only opens up 20% of your cells,

2. Even if after your last step you can't see anything in the tube,

You probably still have lots of good DNA you can do fantastic PCRs with!

You don't know anything about your DNA untill you've redissolved a potentially invisible DNA pellet in H2O (and if you want, measured on a spectrofotometer/nanodrop or run on a gel), and you've used it as template for PCRs.

So if you give us a better problem description and what you've done so far, we can help better.

DNA isolation problems can be due to number of factors, The foremost probability is due to wrong extraction buffer, kindly recheck if everything is ok with the components of your extraction buffer, or else you can make a fresh extraction buffer either with CTAB or 3% SDS. The later gives better results in some cases and The rest of the steps you can follow for Chloroform+isoamyl alcohol washes as per protocols but never forget to add Na Acetate otherwise you will not get precipitation of DNA pellet before putting in chilled ethanol. Soaking of your leaf samples in ethanol for few hrs prior to grinding/ chopping also helps in better lysis of cell wall. DNA isolation for better PCR product and minimal RNA, Phenolics and Polysacharide contamination you can use older leaves rather than young leaves. I have used older leaves for better results in many spp.