Streaking or elongated spot rather than a defined spot?

Possible Solutions:

Sample was overloaded: run the TLC again using a more diluted solution of your sample.
In presence of a base sensitive compound: try to add acetic or formic acid to the eluent (0.1 - 2.0 %).
In presence of an acid sensitive compound: try to add triethylamine to the eluent (0.1 - 2.0 %) or 1 - 10 % ammonia in MeOH / DCM. If it is not working use Alumina as TLC coating.
In presence of too highly polar compounds: try using a specialized silica TLC plate like reversed-phase (C18 for example).

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