Aminos,

What sort of a probe are you using? I suspect that you are using a BBFO or QNP type probe. In these probes, 19F is normally on the inside coil, while 1H is on the outer coil. This will make tuning of 19F more sensitive to sample properties (particularly dielectric constant). Also, the circuitry used to tune the inner coil up to 19F (it is primarily a BB-X coil), might make it more sensitive to capacitive coupling to the sample.

If you use a 1H probe with 1H on the inner coil (e.g. TXI, BBI) then 1H tuning becomes very sensitive to sample properties.

You are doing nothing wrong.

Well, tuning much depends of the solvent you are using as it is 90%+ of your sample. Its dielectric constant influences probe tuning, and also your NMR tubes contributes to that. It is completely all right to have results like this when you tune the probe. Remember that the resonant frequency of an LC circuit depends of the nature of the core of the coil. Tune the probe after every sample change and you should'nt have any problems. The molecule's nature doesen't contribute alot to this, especially if your molecule is diamagnetic. If it's paramagnetic, then it's another story as it will have more influence on the resonant circuit in the probe. As Kirk posted previously, the type of probe will significantly increase the sensitivity on the sample nature when you are tuning. Inverse probes are less sensitive and observe probes are more sensitive. I?m guessing that you have a broadband observe probe if you see such dramatic changes in tuning and matching when exchanging samples.