微信小程序蓝牙写入数据API没有任何回调

let oStr = "FF0004440100000200"  
let buffer = that.string2buffer(oStr)  
wx.writeBLECharacteristicValue({  
    deviceId: that.deviceId,  
    serviceId: that.serviceId,  
    characteristicId: that.writeUUID,  
    value: buffer,  
    success(res) {  
        console.log('写入成功')  
    },  
    fail(res) {  
        console.log('写入失败')  
    },  
    complete(res) {  
        console.log('写入完成')  
    }  
});  

/**  
 * 将字符串转换成ArrayBufer  
 */  
string2buffer(str) {  
    let val = ""  
    if (!str) return;  
    let length = str.length;  
    let index = 0;  
    let array = []  
    while (index < length) {  
        array.push(str.substring(index, index + 2));  
        index = index + 2;  
    }  
    val = array.join(",");  

    // 将16进制转化为ArrayBuffer  
    return new Uint8Array(val.match(/[\da-f]{2}/gi).map(function(h) {  
        return parseInt(h, 16)  
    })).buffer  
},

写入数据后不管是success还是fail还是complete都不执行，我把oStr字符串的长度改成3以内，就会有回调，超过这个长度就没有任何回调。
API上写的是超过20个字节ios没有回调，现在没超过20个字节用的还是安卓手机