Base current for that transistor needs to be ~10% of the collector current for full saturation.
Question is, how much current is the strip channel drawing.
Example:
Strip length is 5meter, and has 60 LEDs/meter.
Strip uses 20mA per three LEDs (one colour).
That's 300/3 = 100*0.02A = 2Amp per colour channel.
That means a transistor base current of 10%, or 0.2Amp, or 200mA.
No Arduino pin can provide that.
Solution is using a darlington (mentioned in StackExchange and in post#13), or a mosfet with very low Vgs(th).
Leo..