That would mean that there are actually no transactions happening during those seconds. With 2 possibilities in mind:You have joined the data feed during a not so active market phase
orYou are testing against the demo account setup of TWS
Take into account that the data feed provided by IB for Forex is not like the data feed for equities, for example. Appearances of the BID price will be recorded as the indication for price oscillation. This is what also others do.
In any case you may also run the sample with --debug and you will have the chance to see all messages delivered by TWS to your client (literally all), which may help you follow the details of the price formation in Forex
You may change BID to ASK or MID (check the IB documentation for possible values) by manipulating the what parameter during the creation of an IBData data feed in backtrader.
Read here: Data feeds reference