Those pre-fabricated LED strip connectors are not guaranteed to be of good quality. Visually inspect and measure resistance from the pins to the solder tabs on the strips (if accessible). A small paper-clip inserted into the connectors is useful for making the connection. Sometimes a resistor shoved into the connector works. Measure between the resistor blob and the connector.
Pro Tip (for real):
Let’s say you have a long wire run and need to test for continuity (broken wire, bad connector), but you don’t have qualified help or expensive tools.
No worries. You can perform checks by yourself with ease.
Start by shoving a resistor so it touches 2 connector pins. Do this 2 times for 3-pin connectors, and 3 times for 4P connectors. Something like this:
──┼──█ 1 █─────┤R1├───┐
──┼──█ 2 █════════════╣
──┼──█ 3 █─────┤R2├───┘
Measure R1 and R2 and note the values.
Take the note (paper or electronic) with you to the other end of the wire or cable.
Measure from pins 1 to 3. Expect resistance value of R1 + R2
Measure from pins 1 to 2. Expect resistance value of R1
Measure from pins 2 to 3. Expect resistance value of R2
Unless there are problems with the cable or connectors, all measurements match expected values.
If you notice unstable readings while jiggling the connections, then the other end of the cable is probably not bad.
If the cable has multiple connection points, you can continue down the cable and repeat the procedure.
It goes pretty fast unless you forget to take the noted R1/R2 values with you…