I use Gate365 regularly as it is an easy way to accumulate Avios, especially when there are double points offers.

However, more often than not, I have to chase outstanding Avios at T+40 as the system is very poor at recognising the transaction, let alone crediting the Avios. John Lewis purchases never seem to appear until I have to chase the Avios.

There are numerous traps one should be aware of falling in to, but the 2 main ones are:
Don't use discount vouchers
Don't do click and collect - always have your order delivered