All i can think of is either you have not installed it correctly or you have inadvertently got mixed up with you're pipes and not installed it to the cold mains feed pipe.
Try turning off the water at the mains Stop c0ck and taking the part that clamps to the pipe off and see if the hole in the pipe has cut through properly. There shoulde be a circular hole of about 8mm cut into the pipe with no obstructions.
Also take into account that these DIY taps are not very good, as they do struggle because of the flow restriction. As you can imagine the water has to squeeze through this small puncture hole, hence the flow is restricted by nearly 50%. Much better to have it plumbed in to the pipes with a 15mm solder Tee fittitng or a compression Tee, thus giving you the maximum flow a 15mm diameter pipe can give.