It’s not necessarily an issue with the app.
Sometimes it’s an issue with the market.
For example, you want to sell but there is no one on the other end who wants to buy.
Another reason could be volatility. If prices are fluctuating rapidly it may be difficult to guarantee the price offered prior to requesting the transaction. Due to lack of guarantee the request is rejected.
It’s worth bearing in mind FT prices are often not accurate on the minute so you should research exact price elsewhere. It’s a slight problem when day trading but FT currently is aimed more toward long term investment. With no fees for trades though I’d say it’s bearable