Have you optimize the PCR conditions for the gene of your interest without SYBR green. How much of the the SYBR green you are using in the reaction it should be around 1X accroding to reaction volume. One more important thing is the MgCl2 concentration generally we use 1-1.5 mM MgCl2 for conditions optimization but when you are doing with SYBR green require some more MgCl2 from 2.5- 3 mM MgCl2. As most of the Real -time Master mix comes with around 3 mM MgCl2. You can try with increase gradual increase in the MgCl2 ( 2- 3 mM).

all the best

Do you reproduce real-time PCR published or do you develop yours?

About the volume of sybr green, suppliers give you some information in their own protocol adaptated to their mixs. Maybe the problem comes from another product such as the primers. Give us an example of your mastermix to better understand your problem

Are you sure that the the amount of SyBr green is the real problem ? Did you try to load your Q-PCR products on a standard ethidium agar gel ? are the amplicons detectable there ?

perform qPCR with SYBR green power plus which contains all the PCR mix, in addition you need to add primer , water , cDNA. The accuracy of your results will improve. Check the melt curve after the qPCR

I usually use 5µl SYBR green (from Roche) for each reaction, in a final volume of 10µl and adjust primers volume. Are you amplifying cDNA or genomic DNA? 

Usually Syber green comes in 2X pack so final volume should be 1X and so if you prepare 10X volume reaction u should have 5ul of syber green in that 

My first advice, check for product, as others said above. 

All dyes inhibit PCR a little, or a lot. But SYBR Green inhibits PCR less than ethidium bromide does when considering molarity and fluorescence, so that's what started the SYBR Green revolution in qPCR--Invented by Carl Wittwer.

It sounds like you could be ordering the dye SYBR Green concentrate http://www.lifetechnologies.com/order/catalog/product/S7563  and adding it to a home brew PCR mix. Back in the old days before there were SYBR Green master mixes, people used to do this all the time. Not any more. If this is what you're doing (diluting the super high concentrated form of the aqueous dye) your dilutions of the original stock could results in SUBSTANTIAL variation, since it requires such a high dilution to be usable in PCR. If you're making your own master mix it could also be a problem of not enough enzyme, you forgot one of the dNTPs, concentration of dNTPs, Mg, pH of the buffer, etc. I suggest a commercial master mix for best reproducibility. If not, you'll just have to control your materials. Remember, there could be royalty implications if you're making your own SYBR Green mix and using it commercially.