This review article makes the point that even the TEGA instrument on Phoenix uses the same technique of heating the samples to a gas state and thereby detecting possible organics and that this method maybe unexpectedly insensitive to organics at low levels because of the interaction with the minerals expected to be on Mars. If you read the review article the impression you get is that the experts on the Viking results expect organics will be detected on the surface if the appropriate sensitive detection methods are used. Note as well that strictly speaking organics have been detected on Mars in the form of methane gas.
It is true that TEGA has not detected organics yet but this could be due to the interaction with those minerals that are on Mars. It should also be noted that the separate Phoenix MECA instrument has not detected the oxidants that were proposed to explain a supposed lack of organics on Mars.
Here's the paper in the Proceedings of the National Academy of Sciences where these conclusions on the possible insensitivity of GCMS type instruments were presented:

The limitations on organic detection in Mars-like soils by thermal
volatilization-gas chromatography-MS and their implications for the
Viking results.
Proceedings of the National Academy of Sciences, October 31, 2006 vol.
103 no. 44 16089-16094
"The failure of Viking Lander thermal volatilization (TV) (without or
with thermal degradation)-gas chromatography (GC)-MS experiments to detect
organics suggests chemical rather than biological interpretations for the
reactivity of the martian soil. Here, we report that TV-GC-MS may be blind to low levels of organics on Mars. A comparison between TV-GC-MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10-90 ìg of refractory or graphitic carbon per gram of soil, which would have been undetectable
by the Viking TV-GC-MS. In iron-containing soils (jarosites from Rio Tinto
and Panoche Valley) and the Mars simulant (palogonite), oxidation of the
organic material to carbon dioxide (CO2) by iron oxides and/or their salts
drastically attenuates the detection of organics. The release of 50-700 ppm of CO2 by TV-GC-MS in the Viking analysis may indicate that an oxidation of
organic material took place. Therefore, the martian surface could have several
orders of magnitude more organics than the stated Viking detection limit.
Because of the simplicity of sample handling, TV-GC-MS is still considered the
standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other
methods to be able to detect extinct and/or extant life."
http://www.pnas.org/content/103/44/16089.full

Two things are very notable here: first, that iron-containing
minerals of the type shown to exist on Mars would have made the GCMS
sensitivity even worse, and second, rather surprisingly, the amounts
of CO2 released on heating in the Viking GCMS might actually have
indicated that organics were present since this is the same response observed in samples with organics on Earth that had the type of iron minerals the MER rovers proved to exist on Mars.

Bob Clark