Not all wires or gel crimps are the same mate,
Possible causes of Gel Crimps failing/melting:
1. Poor connection
The crimping action doesn’t always create the desired connection, especially when you are on site, or in a rush in the workshop. You may find when you tested the sign it was all good. But you can’t actually see the connection ie, if only a very small section of the outer PVC insulation was stripped in the crimping process then the conduction would be poor. This will eventually cause overheating and melt the copper strand/s which were conducting electricity. Its a good idea to check the gel all gel crimps are bedded flat to the top surface of the crimp body.
2. Overload using incorrect wire gauge
I hear a lot of the time, sign makers using speaker or bell wire for connecting LED strings together and more dangerously using the speaker wire to make connections of multiple LED circuits to a single LED Driver. If the Driver is not fitting within the sign, typically fitters will combine the circuits close to the LEDs into a connector block and then route a twin core flex to the LED Driver. This section of the wire must be of a higher rating than the wire used to join single LED strings (depending on the no. of LEDs and the load in total). Wires have specific maximum load amperage ratings which should not be exceeded. If exceeded the wire will most likely melt. Using gel crimps limits your choice due to the size of the holes in the crimps, hence always us the the thickest guage wire possible, from memory the gel crimp in your photo will take a 0.75mm CSA wire with a max 2.08mm outer diameter. 0.75mm is typically about 6Amp so the total permissible load at 12V would be 72W
Also be warned that cheap wire is Copper Clad Aluminium (CCA) this wire is has inferior conductivity to solid copper stranded wire, and hence more likely to melt. Cheap speaker wire is made from CCA
3. Gel Crimps
Unknown to most, Gel Crimps were designed for low voltage telecommunications use, These are not rated nor are they recommend for 12V or 24V signage applications. BUT we see them doing a good job all the same most of the time when used with care, as mentioned in points above. 3M UR2 and 3M UY2 are better to use, but again be warey there is alot of counterfeit substandard chinese knockoffs on ebay and amazon. I would recommend 3M/Corning packaged product only.
How many LEDs are you connecting to the Gel Crimp? Do you have a photo of the burnt out gel crimp in order to identify the cause from the points mention above.
I hope it helps.