Start with the numbers
A Class A GFCI trips somewhere between 4 and 6 mA of imbalance. Every appliance with a mains filter leaks a little current to ground through its capacitors, and the product standards allow it: roughly half a milliamp for most portable appliances, up to about three quarters for some. Plug six or seven of them into one GFCI cord and the leakage alone can reach the trip level with nothing faulty anywhere. That is the arithmetic behind a lot of "it trips when I add the last thing".
The checklist, in order
Unplug everything from the cord. Press RESET. If it will not hold with nothing plugged in, the fault is in the cord or the device: a wet connector, a cut jacket, water inside the housing, or a failed device. Dry it, look it over, test with the button (guide 6). If it still will not hold, replace it.
Add loads back one at a time. The one that trips it is your suspect. Run that one on a known-good GFCI outlet indoors; if it trips there too, the appliance is leaking and needs repair, not the cord.
Look for water. Connectors lying in a puddle, a cord end that was rained on, a tool that was hosed off. Water across the pins of a connector is a leakage path from hot to ground, and an in-line GFCI will trip on it every time the sun goes in. Lift connectors off the ground, use covers, and dry things before you blame anything electrical.
Heating elements. Mineral-insulated elements (water heaters, deicing cable, some space heaters, sterilizers, coffee urns) absorb moisture when cold and leak until they have warmed up and dried. A heater that trips the GFCI in the first minute and then holds is telling you its element is damp or its sheath is cracked. Run it on an unprotected outlet only long enough to prove the theory, then repair it.
Motors. A healthy motor does not trip a GFCI. A motor with worn brushes, a wet winding or a failing capacitor does, and often only at start-up when the insulation is stressed. Pumps that sit in water are the usual suspects. Universal-motor tools that have been dropped come next.
Electronics and drives. Variable-speed drives, EV chargers, LED drivers and switch-mode supplies leak through their filters and can put DC or high-frequency components on the line. On the IEC side this is why RCD Type A and Type B exist (guide 2). On a UL 943 GFCI, several of these on one cord is the leakage arithmetic again; spread them over more than one protected cord.
A second neutral-to-ground bond. If the GFCI trips as soon as it is connected to a generator, an RV, a boat, or a machine with its own panel, some of the neutral current is returning on the ground conductor. The GFCI is correct to trip. Find the extra bond and remove it (guide 5).
Shared neutrals. A GFCI receptacle wired into a multi-wire branch circuit whose neutral is shared with another circuit downstream will trip constantly. This is a wiring fault, not a cord fault, and it needs an electrician.
Cord length. A long cord does add a little capacitive leakage, and a long damp cord adds more, but a sound 100 ft cord on its own does not trip a GFCI. If length seems to be the variable, the extra length is usually lying in the wet somewhere.
Only now, the device. If the cord holds with nothing plugged in, passes the button test, and trips only with a load that runs fine on another GFCI, the device may be tripping early. Swap it for a known-good GFCI cord and see. Devices do drift, and a cheap one drifts sooner.
What not to do
Do not swap the GFCI cord for a plain cord "just until the job is done". Do not bypass the device. Do not tape the RESET button. A GFCI that trips is doing the one thing you bought it for, and the trip is information. If a site genuinely has a load that cannot live with 5 mA protection, the answer is to fix the load or to change the design, not to remove the protection.
Automatic reset is not a fix for tripping
An automatic-reset device (guide 3) will reconnect after a power loss, but a real ground fault holds it open the same as a manual device. Buying automatic reset to cure nuisance tripping just hides the trips from you.